"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).

"once a month, in every month," 1530s, from month + -ly (2).

1570s, "pertaining to or relating to a month," from month + -ly (1). Old English had monaþlic, but the modern words seem to be separate formations. From 1580s as "continued for a month;" 1640s as "done or happening once every month." As a noun, "a magazine or periodical published once a month," by 1833.

late 14c., "pertaining to menses of females," from Old French menstruel and directly from Latin menstrualis "monthly," especially "of or having monthly courses," from menstruus "of a month, every month, monthly, pertaining to a month," from mensis "month" (see moon (n.)). Occasionally, in astronomy, "monthly" (1590s).

"having the monthly flow or discharge," early 15c., from Old French menstrueus and directly from Medieval Latin *menstruosus, from Latin menstruum "of or belonging to menstruation," from menstruus (adj.) "that happens every month, monthly," from mensis "month" (see moon (n.)).