I looked the term up on Urban Dictionary, which got to the point:
"To explain in a patronizing manner, assuming total ignorance on the part of those listening. The mansplainer is often shocked and hurt when their mansplanation is not taken as absolute fact, criticized or even rejected altogether."
Right, I'm far more open to that. Although I can't see why anything in there can't apply to women also. Do we also need a 'femsplain', 'gaysplain', 'transplain', 'Tory-splain', 'Marx-splain', etc?