Clauson (1972)
An etimological dictionary of pre-thirteenth-century Turkish
p. 823
VUC seksö:n
‘eighty’; crasis of sekiz o:n, which was the form used in Türkü viii, Ix. 3 and Uyğ. viii ff. Bud., PP 24, 4 and still survives as segiz on in NE Khak. In all other modern languages the form is seksen or the like. Xak. xi seksö:n the number ‘eighty’; originally sekiz o:n ‘eight times ten’ and then joined together (cu‘ilā wāḥid) Kaş. I 437: xiii Tef. seksen ‘eighty’ 267: xiv Muh. ‘eighty’ sekze:n Mel. 81, 15; seksem Rif. 187: Xwar. xiv seksen ‘eighty’ Qutb 156: Kıp. xiii ditto Hou. 22, 15: xiv ditto İd. 53; Bul. 12, 14: xv ditto Kav. 39, 7; 65, 9 (seksem); Tuh. 60b. 10.