1. 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.Yes. Mansplaining does have echoes in the world of music, particularly when it comes to the evaluation of newly-composed music or the relative importance of music written by women (either living or dead). I hear and read those echoes and utterances regularly. Of course not every musically-inclined or musically-informed man is a mansplainer, and, for that matter, not everyone who does it is a man, but the practice keeps rearing its ugly head, generation after generation.
2. Despite claims of superior strength in avoiding over-emotional reactions, when a man encounters even one iota of criticism of men on the internet, he must then mansplain why women suck by comparison or must be radical feminists.
3. To explain something in an unnecessarily long winded way, so as to dominate the conversation, and to make statements that are not based on facts, assuming that people will believe and agree with him because he is male.
4. Delighting in condescending, inaccurate explanations delivered with rock solid confidence of rightness and that slimy certainty that of course he is right, because he is the man in this conversation.
5. 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.
Here's an article about mansplaining and the way Mormon men react to Mormon women who are pro ERA. It has an excellent comment section and a full array of links, including one to Rebecca Solnit's article on the subject in Mother Jones.