OCTOBER. In the ancient Roman calendar, October was the name of the eighth month of the year. Its name comes from octo, the Latin word for “eight.” When the Romans converted to a 12-month calendar, they tried to rename this month after various Roman emperors, but October’s name stuck!late Old English, from Latin October (mensis), from octo “eight,” from PIE root *octo(u)- “eight” (see eight). The eighth month of the old Roman calendar (pre-46 B.C.E.), which began the year in March. For -ber see December. Replaced Old English winterfylleð. In Russian history, the October Revolution (in which the Bolsheviks overthrew the Provisional Government) happened Nov. 7, 1917, but because Russia had not at that time adopted the Gregorian calendar reform, this date was reckoned there (Old Style) as Oct. 25.
“twelfth and last (by modern reckoning) month of the calendar, the month of the winter solstice,” late Old English, from Old French decembre, from Latin December, from decem “ten” (from PIE root *dekm- “ten”); tenth month of the old Roman calendar, which began with March.
The -ber in four Latin month names is probably from -bris, an adjectival suffix. Tucker thinks that the first five months were named for their positions in the agricultural cycle, and “after the gathering in of the crops, the months were merely numbered.”
If the word contains an element related to mensis, we must assume a *decemo-membris (from *-mensris). October must then be by analogy from a false division Sep-tem-ber &c. Perhaps, however, from *de-cem(o)-mr-is, i.e. “forming the tenth part or division,” from *mer- …, while October = *octuo-mr-is. [T.G. Tucker, “Etymological Dictionary of Latin”]”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.
Decembrist, in Russian history in reference to the insurrection against Nicholas I in December 1825, is by 1868 in English, translating Russian dekabrist, from dekabr’ “December.”