I don't think 'mansplaining' is a very good term, I'm sure some women also speak this way so it might be better to come up with a term that covers the behaviour rather than the gender of the person speaking. To me it sounds sewing the seeds for stereotyping eventually leading to prejudice.
Oh, "obnoxious explaining" is a thing for both genders. I'm quite fond of obnoxiousness myself! But mansplaining refers specifically to men who feel like when they're in a conversation with a woman, they have to explain the basics of whatever topic they're discussing, even if the person they're talking to is much more experienced with said topic than the man is.
For instance, a lot of this conversation has revolved around men explaining why they think a policy against harassment at conferences is a bad thing. We don't have a lot of women participating in this discussion, but I suspect that if they did, they'd garner plenty of responses saying "Clearly you don't understand this issue as well as I[, a man,] do." And I suspect that because when topics like this on HN do get responses from posters that go "I'm a woman and I think this is helpful," they inevitably receive that exact kind of response.
I've been told, by some of my close ladyfriends, that this behavior is enormously frustrating, and that most of them recognize it as a thing which is almost always what a guy does to a woman, rather than a guy to a guy or a woman to a man. I see enough of that behavior to agree with them that yes, the term has a legitimate use (though it doesn't preclude people of either gender from being more generally irritating, of course).