eighth adj., n.

"1 more than seven, twice four; the number which is one more than seven; a symbol representing this number;" late 14c., eighte, earlier ehte (c. 1200), from Old English eahta, æhta, from Proto-Germanic *akhto (source also of Old Saxon ahto, Old Frisian ahta, Old Norse atta, Swedish åtta, Dutch acht, Old High German Ahto, German acht, Gothic ahtau), from PIE *okto(u) "eight" (source also of Sanskrit astau, Avestan ashta, Greek okto, Latin octo, Old Irish ocht-n, Breton eiz, Old Church Slavonic osmi, Lithuanian aštuoni). From the Latin word come Italian otto, Spanish ocho, Old French oit, Modern French huit. For spelling, see fight (v.).

Meaning "eight-man crew of a rowing boat" is from 1847. The Spanish piece of eight (1690s) was so called because it was worth eight reals (see piece (n.)). Figure (of) eight as the shape of a race course, etc., attested from c. 1600. To be behind the eight ball "in trouble" (1932) is a metaphor from shooting pool. Eight hours as the ideal length of a fair working day is recorded by 1845.

word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins].

Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek tēlikos "so old, of such an age," Old Church Slavonic toli "so, to such a degree," toliku "so much," Russian toliko "only;" also see -ed.