Clauson (1972)
An etimological dictionary of pre-thirteenth-century Turkish
p. 473-474
C tokso:n
(d-) crasis of tokkuz o:n, which phr. was used down to Uyğ. inclusive; ‘ninety’. S.i.a.m.l.g. usually as toksan; SW Az. doxsan; Osm. doksan; Tkm. toksan (although ‘nine’ dokkiz). An important word as giving an incontrovertible example of modern -a- representing earlier -o-. Xak. xi tokso:n the numeral ‘ninety’, originally tokuz o:n that is ‘nine times ten’ and then combined (cu‘ilata zawāhid) Kaş. I 437: xiii(?) Tef. toksan: xiv Muh. tis‘ūna dokuzan Mel. 81, 15; toksan Rif. 187: Kıp. xiii tis‘īna toksan Hou. 22, 7: xiv toksan ditto İd. 65; toksan Bul. 12, 14: xv toksan Kav. 4, 21; a.o.o.; Tuh. 60b. 10.