Earlier quoted context omitted.
Of all terms that have been formed on the internet, "mansplaining" is the one I hate the most. All a discussion is, at the end of the day, is two people explaining their own viewpoints. But now, with this word, you have a get-out-of-conversation-free card because if you don't like someone's response, just say it's mansplaining. It's almost unbelievably ignorant.
I can't speak for the abuse of the term (I've never experienced what you're describing), but 'mansplaining' refers to a very real phenomenon. To put gender aside, egos and biases can always get in the way of two people explaining their viewpoints. You are pretty lucky if you've never had somebody explain a concept you already understood in a very condescending way. Many men do this to women when they assume that thes…
But first, men do this to each other all the time. Most of my conversations with my male friends involve us constantly disagreeing with each other, in a way I've rarely observed in groups of women. It's far more likely that if someone is willing to be condescending, they're going to do it to everyone.
Of course there are instances where a man may only do it to women, but we've quickly moved on from that general observation to "any time a man disagrees with a women he's mansplaining". 99% of the time I've seen this term used, it's to lazily drop out of a discussion once it becomes difficult.