Why is the ninth month called September?
1520s, likely from an unrecorded Middle Low German *moneke or Middle Dutch *monnekijn, a colloquial word for "monkey," originally a diminutive of some Romanic word, cf. French monne (16c.); Middle Italian monnicchio, from Old Italian monna; Spanish mona "ape, monkey." In a 1498 Low German version of the popular medieval beast story "Roman de Renart" ("Reynard the Fox"), Moneke is the name given to the son of Martin the Ape; transmission of the word to English might have been via itinerant entertainers from the German states.
The Old French form of the name is Monequin (recorded as Monnekin in a 14c. version from Hainault), which could be a diminutive of some personal name, or it could be from the general Romanic word, which may be ultimately from Arabic maimun "monkey," literally "auspicious," a euphemistic usage because the sight of apes was held by the Arabs to be unlucky [Klein]. The word would have been influenced in Italian by folk etymology from monna "woman," a contraction of ma donna "my lady."
Monkey has been used affectionately for "child" since c.1600. As a type of modern popular dance, it is attested from 1964. Monkey business attested from 1883. Monkey suit "fancy uniform" is from 1886. Monkey wrench is attested from 1858; its figurative sense of "something that obstructs operations" is from the notion of one getting jammed in the gears of machinery (cf. spanner in the works). To make a monkey of someone is attested from 1900. To have a monkey on one's back "be addicted" is 1930s narcotics slang, though the same phrase in the 1860s meant "to be angry." There is a story in the Sinbad cycle about a tormenting ape-like creature that mounts a man's shoulders and won't get off, which may be the root of the term. In 1890s British slang, to have a monkey up the chimney meant "to have a mortgage on one's house." The three wise monkeys ("see no evil," etc.) are attested from 1926.
1859, "to mock, mimic," from monkey (n.). Meaning "play foolish tricks" is from 1881. Related: Monkeyed; monkeying.