It's a gender-specific putdown, like saying a woman is emotional, or a black person "uppity."
Not only is it rude, unfair, and a lot of other negative things, it also says that the accuser has no real objection to what the person is saying. If they did, why resort to an ad hominem attack.
If you're being lectured at, condescended to, how about walking away?
The term is also wildly misused. How could a blog post be mansplaining? Yet I've heard it said many times that a blog post is. (You're not cornered, you can always hit the Back button, and it's not making any assumptions about gender of the reader, blog posts are read by people of all genders, races, ages etc.)
Women do it too.
If the term is a feminist ideal, to label a bad behavior so people can see it, it has backfired. The term is most often used to shut people up, to shame them.