A word typically used to imply that the previous speaker is overly condescending and/or has a different way of explaining because he happens to be a man and the other party a woman. Usually, this is based in an over- or misinterpretation based on prejudice about the alleged mainsplainers intentions or on an inability to take constructive feedback. Sometimes it is used as an attempt to unfairly discredit the speaker or his arguments without having to provide counter-arguments. Typically, the mansplainer is simultaneously implied to be sexist or misogynistic.
Explainee: Women only earn 80 cents for every dollar a man makes. This is a grave injustice.
Explainer: That is a misleading statement. In fact, after correcting for factors like hours worked, experience, physical danger involved, etc., the differences is just a few percent.
Explainee (sarcastic): Thank you for mansplaining! I would never have thought of that with my tiny little brain!