Why is the ninth month called September?
"female horse," Old English mere (Mercian), myre (West Saxon), fem. of mearh "horse," from Proto-Germanic *markhjon- (cf. Old Saxon meriha, Old Norse merr, Old Frisian merrie, Dutch merrie, Old High German meriha, German Mähre "mare"), said to be of Gaulish origin (cf. Irish and Gaelic marc, Welsh march, Breton marh "horse"). No known cognates beyond Germanic and Celtic. As the name of a throw in wrestling, it is attested from c.1600. Mare's nest "illusory discovery, excitement over something which does not exist" is from 1610s.
"broad, dark areas of the moon," 1765, from Latin mare "sea" (see marine), applied to lunar features by Galileo and used thus in 17c. Latin works. They originally were thought to be actual seas.
"night-goblin, incubus," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" (cf. Middle Low German mar, Middle Dutch mare, Old High German mara, German Mahr "incubus," Old Norse mara "nightmare, incubus"), from PIE *mora- "incubus" (cf. first element in Old Irish Morrigain "demoness of the corpses," literally "queen of the nightmare," also Bulgarian, Serbian mora, Czech mura, Polish zmora "incubus;" French cauchemar, with first element from Old French caucher "to trample"), from root *mer- "to rub away, harm" (see morbid).
Plural maria (mä'rē-ə)
Any of the large, low-lying dark areas on the Moon or on Mars or other inner planets. The lunar maria are believed to consist of volcanic basalts, and many are believed to be basins formed initially by large impacts with meteoroids and later filled with lava flows. Compare terra.