The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
okt(u)
DEFINITION:
Eight. Oldest form *ot(u), becoming *okt(u) in centum languages. 1a.eight, eighteen, eighty, from Old English eahta, eight, with derivatives eahtatig, eighty, and eahtatne, eighteen (-tne, ten; see dek); b. atto-, from Old Norse ttjn, eighteen (tjn, ten; see dek). Both a and b from Germanic *aht.2.Octans, octant, octave, octavo, octet, octo-, October, octonary; octodecimo, octogenarian, from Latin oct, eight. 3.octad, octo-; octopus, from Greek okt, eight. 4.at2, from Sanskrit a, eight. (Pokorny ot(u) 775.)
(u)
t
ne, eighteen (-t
); b. atto-, from Old Norse 
ttj
