According to the Ukrainian-American professor Omeljan Pritsak the word 'Slav, Sklavin' is Proto-Bulgairan, i.e. Turkic, Volgo-Tataric in origin:
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Omeljan Pritsak, The Slavs and the Avars (p. 405-407 in SETTIMANE DI STUDIO DEL CENTRO ITALIANO DI STUDI SULL'ALTO MEDIOEVO, XXX, 1983
http://www.kroraina.com/slav/op/op_slavs_avars.htm
...
Since all attempts to find an etymology of the term Sklavin- / Slav-, on native ground have failed, one is tempted to look elsewhere [137]. Proto-Bulgarian seems the most promising spot. There we find a common Hunno-Turkic word saqla-, 'to watch over, guard, protect' [138]. The noun derived from it by the suffix */GU/ is attested in Kazan-Tatar (Muslim progeny of the Volga Bulgars) and in Karaim (modern Qipčaq-Polovcian), where the suffix became /-w/. In these languages the noun saqla-w means 'guard, watch; guarding' in the senses of actor, profession, place, or action [139]. As early as Proto-Bulgarian, the suffix */GU/ had become /w/ : e.g., κολο-β-ρ (< *qola-ġu-r) 'leader' [140]. Further, in Proto-Bulgarian stress moved from the root syllable to the suffix, and the root vowel then reduced, e.g., *dawl-an > dwan 'hare', *tovirəm > tvirəm «the ninth» [141]. Therefore one can assume that in Proto-Bulgarian the old *saqla-ġu would develop as *saqla-w and later as sqlaw-. Proto-Bulgarian also had a collective suffix /-in/, used especially to designate peoples: e.g., Volga Bulgarian Bulgar-in, «the Bulgars», Sowar-in «the Sowars» [142].
Thus our conclusion is that there was a Proto-Bulgarian word saqlaw > sqlaw with the plural form *sqlaw-in and two meanings: 1) «guard, watch, guarding»; 2) «trained slave». The Arabs, who were engaged in the slave trade, (see below), adopted the singular form as ṣ(a)qlab, meaning «trained slave», while the Byzantines, who were interested in contacts with the collective of the sqlawin on their limes, adopted it as sklavin, adding a plural desinence: Σκλαβην-οί. In Slavic, the suffix was modified to the collective plural -ěn-e, denoting a social group, correlated with the singulative suffix -in-, while the impermissible initial cluster *skl was reduced to sl.
(137) For bibliography, see Leszek Moszyński's (unsatisfying) essay, «Czy Słowianie to rzeczywiście nomen originis»? Z polskich studiów slawistycz nych. Seria V, Warsaw 1978, 499-507. Tuomo Pekkanen («L'origine degli Slavi e il loro nome nella letteratura greco-latina», Quaderni Urbinati, N. 11, 1971, pp. 51-64) suggests Slavic slab- 'weak' (implausible), and Georg Kobth («Zur Etymologie des Wortes 'Slavus' (Sklave)», Glotta 48 [Göttingen 1970], 145-153), starting from the meaning «slave», posits a linguistically and sociologically unlikely derivation from Greek σκύλον «Kriegsbeute».
(138) See the data in Clauson, Etym. dict., pp. 803, 810, and Martti Räsänen, Versuch eines etymologischen Wörterbuchs der Türksprachen, Helsinki 1969, pp. 395-396. The verb is of denominal origin (saq). Kāšġarī (ca. 1070) explains the meaning of the etymon saq as follows: «saq saq» an exclamation (ḥarf) used by a sentry (al-ḥāris) in the army to order alertness (al-tayaqquẓ) to protect castles, forts, or horses from the hands of the enemy; one says saqsaq «be alert (ayqāẓ)»; hence one calls «an intelligent, (alert) man (al-faṭinu'l-mutayyaqiẓ)» saq är [är «man»], facsimile ed. by Atalay, pp. 167-168.
(139) Kazan-Tatar saqla-u «Bewahren, Behüten» (Wilhelm Radloff, Versuch eines Wörterbuches der Türk-Dialecte, vol. 4, [repr. Hague 1960], col. 252); Karaim Troki saqla-w «die Wache», ibid. col. 254; Karaim Luc'k saqlaw «die Wache» (Aleksander Mardkowicz, Słownik karaimski [Luc'k 1935], p. 55); cf. Karaimsko-russko-pol'skij slovar', Nikolaj Aleksandrovič Baskakov et al., eds., Moscow 1974, p. 461: saqlaw «1. oxrana, straz; 2. xranenie». See also Chaghatai saqlau «die Kriegsgeisel», Radloff, Wb., col. 252.
Concerning the deverbal nominal sufïix /GU/, which has three meanings, 1) actor; 2) abstracts; 3) instruments: see Annemarie von Gabain, Alttürkische Grammatik, 2nd ed., Leipzig 1950, pp. 71-72; Ananiasz Zajączkowski, Sufiksy imienne i czasownikowe w języku zachodniokaraimskim, Cracow 1932, pp. 66-68; Èrvand Vladimirovič Sevortjan, Affiksy imennogo slovoobrazovanija v azerbajdžanskom jazyke, Moscow 1966, pp. 227-232.
(140) See also the Volga Bulgarian inscription from 1307: belü «sepulchral monument» < *bälgü; cf. Old Turkic bälgü; see also A. Róna-Tas, Acta Orientalia 30 (1976) 159.
(141) Pritsak, Bulgarische Fürstenliste, pp. 46, 56-58.
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Omeljan Pritsak, The Slavs and the Avars (p. 405-407 in SETTIMANE DI STUDIO DEL CENTRO ITALIANO DI STUDI SULL'ALTO MEDIOEVO, XXX, 1983
http://www.kroraina.com/slav/op/op_slavs_avars.htm
...
Since all attempts to find an etymology of the term Sklavin- / Slav-, on native ground have failed, one is tempted to look elsewhere [137]. Proto-Bulgarian seems the most promising spot. There we find a common Hunno-Turkic word saqla-, 'to watch over, guard, protect' [138]. The noun derived from it by the suffix */GU/ is attested in Kazan-Tatar (Muslim progeny of the Volga Bulgars) and in Karaim (modern Qipčaq-Polovcian), where the suffix became /-w/. In these languages the noun saqla-w means 'guard, watch; guarding' in the senses of actor, profession, place, or action [139]. As early as Proto-Bulgarian, the suffix */GU/ had become /w/ : e.g., κολο-β-ρ (< *qola-ġu-r) 'leader' [140]. Further, in Proto-Bulgarian stress moved from the root syllable to the suffix, and the root vowel then reduced, e.g., *dawl-an > dwan 'hare', *tovirəm > tvirəm «the ninth» [141]. Therefore one can assume that in Proto-Bulgarian the old *saqla-ġu would develop as *saqla-w and later as sqlaw-. Proto-Bulgarian also had a collective suffix /-in/, used especially to designate peoples: e.g., Volga Bulgarian Bulgar-in, «the Bulgars», Sowar-in «the Sowars» [142].
Thus our conclusion is that there was a Proto-Bulgarian word saqlaw > sqlaw with the plural form *sqlaw-in and two meanings: 1) «guard, watch, guarding»; 2) «trained slave». The Arabs, who were engaged in the slave trade, (see below), adopted the singular form as ṣ(a)qlab, meaning «trained slave», while the Byzantines, who were interested in contacts with the collective of the sqlawin on their limes, adopted it as sklavin, adding a plural desinence: Σκλαβην-οί. In Slavic, the suffix was modified to the collective plural -ěn-e, denoting a social group, correlated with the singulative suffix -in-, while the impermissible initial cluster *skl was reduced to sl.
(137) For bibliography, see Leszek Moszyński's (unsatisfying) essay, «Czy Słowianie to rzeczywiście nomen originis»? Z polskich studiów slawistycz nych. Seria V, Warsaw 1978, 499-507. Tuomo Pekkanen («L'origine degli Slavi e il loro nome nella letteratura greco-latina», Quaderni Urbinati, N. 11, 1971, pp. 51-64) suggests Slavic slab- 'weak' (implausible), and Georg Kobth («Zur Etymologie des Wortes 'Slavus' (Sklave)», Glotta 48 [Göttingen 1970], 145-153), starting from the meaning «slave», posits a linguistically and sociologically unlikely derivation from Greek σκύλον «Kriegsbeute».
(138) See the data in Clauson, Etym. dict., pp. 803, 810, and Martti Räsänen, Versuch eines etymologischen Wörterbuchs der Türksprachen, Helsinki 1969, pp. 395-396. The verb is of denominal origin (saq). Kāšġarī (ca. 1070) explains the meaning of the etymon saq as follows: «saq saq» an exclamation (ḥarf) used by a sentry (al-ḥāris) in the army to order alertness (al-tayaqquẓ) to protect castles, forts, or horses from the hands of the enemy; one says saqsaq «be alert (ayqāẓ)»; hence one calls «an intelligent, (alert) man (al-faṭinu'l-mutayyaqiẓ)» saq är [är «man»], facsimile ed. by Atalay, pp. 167-168.
(139) Kazan-Tatar saqla-u «Bewahren, Behüten» (Wilhelm Radloff, Versuch eines Wörterbuches der Türk-Dialecte, vol. 4, [repr. Hague 1960], col. 252); Karaim Troki saqla-w «die Wache», ibid. col. 254; Karaim Luc'k saqlaw «die Wache» (Aleksander Mardkowicz, Słownik karaimski [Luc'k 1935], p. 55); cf. Karaimsko-russko-pol'skij slovar', Nikolaj Aleksandrovič Baskakov et al., eds., Moscow 1974, p. 461: saqlaw «1. oxrana, straz; 2. xranenie». See also Chaghatai saqlau «die Kriegsgeisel», Radloff, Wb., col. 252.
Concerning the deverbal nominal sufïix /GU/, which has three meanings, 1) actor; 2) abstracts; 3) instruments: see Annemarie von Gabain, Alttürkische Grammatik, 2nd ed., Leipzig 1950, pp. 71-72; Ananiasz Zajączkowski, Sufiksy imienne i czasownikowe w języku zachodniokaraimskim, Cracow 1932, pp. 66-68; Èrvand Vladimirovič Sevortjan, Affiksy imennogo slovoobrazovanija v azerbajdžanskom jazyke, Moscow 1966, pp. 227-232.
(140) See also the Volga Bulgarian inscription from 1307: belü «sepulchral monument» < *bälgü; cf. Old Turkic bälgü; see also A. Róna-Tas, Acta Orientalia 30 (1976) 159.
(141) Pritsak, Bulgarische Fürstenliste, pp. 46, 56-58.
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