Comment by scarmig

It originated with Rebecca Solnit, who had an experience where a man and she were talking and a book came up that he had read. As he's telling her about the book, she points out that she's the author of the book, and he proceeds to continue explaining the book to her.

I certainly see this happening sometimes. Earlier this month I remember sitting in on a meeting where a boy fresh out of college was trying to explain SQL injection to a (female) senior security engineer.

That said, I think that happens to everybody, albeit disproportionately to women. And now mansplaining has other really silly uses as an epithet, like "a male with expertise is telling me something I don't want to hear" or "a male disagrees with something I believe very strongly."