A man whom by virtue of the authority and privilege vested in him by society feels entitled to preach or explain how the world works. Bonus, if he is speaking to women, whom he perceives to be naive and ignorant, about problems and concerns with which women have real life every day experience, and the man has NO fucking clue what he is talking about. This is usually done in a patronizing insensitive manner.
A mansplainer lectures on gynecology, menstruation or gender oppression to women.
Ex. I think women should...
The problem with women today is...