Old English mennisc "human, human-like, natural," from Proto-Germanic *manniska- (cf. Old Saxon mannisc, Old High German mennisc, Gothic mannisks), from *manna- (see man (n.)). In some cases a new formation from man (n.) + -ish.
Sense of "masculine" is from late 14c.; in reference to women seen as masculine, from late 14c. Of adult males (opposed to childish) from 1520s. Related: Mannishly; mannishness. The proto-Germanic adjective became, in some languages, a noun meaning "human" (cf. German Mensch), and in Old English mannish also was used as a noun "mankind, folk, race, people."