"one-twelfth part of a year; one of the twelve parts into which the calendar year is arbitrarily divided," Old English monað, from Proto-Germanic *menoth- (source also of Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), which is related to *menon- "moon" (see moon (n.)). Originally the month was the interval between one new moon and the next (a sense attested from late Old English).
Its cognates mean only "month" in the Romance languages, but in Germanic they generally continue to do double duty. The development of the calendrical meaning for words from this root in Greek (mēn) and Latin (mensis) was accompanied by the creation of new words for "moon" (selēnē, luna). The phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months but never used literally).