Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., eighte, earlier ehte (c.1200), from Old English eahta, æhta, from Proto-Germanic *akhto(u) (cf. 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" (cf. Sanskrit astau, Avestan ashta, Greek okto, Latin octo, Old Irish ocht-n, Breton eiz, Old Church Slavonic osmi, Lithuanian aštuoni).
Klein calls it "an old dual form, orig. meaning 'twice four.' " 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.