Mansplaining refers to when a male explaining on something to a woman in a condescending, overconfident, and often inaccurate or oversimplified manner, usually coming from a state of overconfidence and cluelessness.

It is a blend of the word "man" and the informal "splaining" of the verb "explaining," for example, "Did you hear him try mansplain that to me? What a 404."
NetLingo Classification: Online Jargon