Etymology
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eight (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.

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octo- 

word-forming element, before vowels oct-, from combining form of Latin octo "eight," from PIE root *octo(u)- "eight" (see eight). Words made from Greek elements or derived from Greek typically are octa-.

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octoroon (n.)

"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.

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eighty (adj., n.)

"1 more than seventy-nine, twice forty; the number which is one more than seventy-nine; a symbol representing this number;" late 13c., eigteti, from eight + -ty (1). Replacing Old English hundeahtatig, with hund- "ten." Related: Eightieth.

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octa- 
before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.
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octavo (n.)

1580s, printer's word for sheets folded to make eight leaves, from Latin in octavo "in the eighth," ablative of octavus "eighth" from octo "eight" (see eight, and compare octave). Abbreviation is 8vo.

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eighteenth (adj., n.)

"next in order after the seventeenth; an ordinal numeral; being one of eighteen equal parts into which a whole is regarded as divided;" mid-13c., egtetenþe, modified, by influence of eighteen, from Old English eahtateoða; from eight + teoða "tenth" (see -ty (1)). Cognate with German achtzehnte, Danish attende, Swedish adertonde.

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eighth (adj., n.)

"next in order after the seventh; an ordinal numeral; being one of eight equal parts into which a whole is regarded as divided;" late 14c., eighthe, contracted from Old English eahtoða, from Proto-Germanic *ahtudon (source also of Old High German ahtoda, Old Frisian achta, German achte, Gothic ahtuda);see eight + -th (1).

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eighteen (adj., n.)

"1 more than seventeen, twice nine; the number which is one more than seventeen; a symbol representing this number;" late 14c., eightene, earlier ahtene (c. 1200), from Old English eahtatiene, eahtatyne; see eight + -teen. Cognate with Old Frisian schtatine, Old Saxon ahtotian, Dutch achttien, Old High German ahtozehan, German achtzehn, Old Norse attjan, Swedish adertån.

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