Old English mennisc, mænnisc "human, human-like, natural to the human species," from Proto-Germanic *manniska- (source also of Old Saxon mannisc, Old High German mennisc, Gothic mannisks), from *manna- (from PIE root *man- (1) "man"). In some cases a new formation from man (n.) + -ish.
Sense of "masculine, characteristic or resembling the males of the human kind" is from late 14c.; also from late 14c. in reference to women seen as masculine. As "characteristic of a grown man" (opposed to childish) from 1520s. Related: Mannishly; mannishness. The Proto-Germanic adjective became, in some languages, a noun meaning "human" (such as German Mensch), and in Old English mannish also was used as a noun "mankind, folk, race, people."