A term created by radical feminists to automatically discredit the opinion of a man because he's, well, a man. See also Feminist Shaming Language.
Man: I think your argument about women being oppressed by an evil patriarchy is wrong because of x, y and z.
Feminist group: Oh look, mansplaining from a member of the male species! How dare you interrupt our bitching session with your facts and logic! Let's get him, sisters! Make jokes about him having a small penis, that'll show him!