"offspring of a quadroon and a white," 1861, an irregular formation from Latin octo "eight" (see eight) + suffix abstracted from quadroon (in which the suffix actually is -oon). So called for having one-eighth Negro blood.
"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.
by 1781, an alteration (by influence of words in quadr-) of quarteroon (1707), "offspring of a white and a mulatto," from Spanish cuarteron (used chiefly of the offspring of a European and a mestizo), literally "one who has a fourth" (Negro blood), from cuarto "fourth," from Latin quartus "the fourth, fourth part," which is related to quattuor "four" (from PIE root *kwetwer- "four").
So called because he or she has one quarter African blood. There also was some use in 19c. of quintroon (from Spanish quinteron) "one who is fifth in descent from a Negro; one who has one-sixteenth Negro blood." OED lists quarter-caste as an Australian and New Zealand term for a person whose ancestry is one-quarter Aboriginal or Maori and 3/4 white (1948).