I think it is meant to pertain to condescension, particularly the assumption that a man is condescending to a woman by assuming she is ignorant for being a woman.
However, one person may not know what another does or does not know. While it may be more tactful to ask about the knowledge of the other to better understand what information may or may not be missing, it is more efficient to simply break down the subject to its most basic parts and build up from there to the conclusion.

That said, while mansplaining is condescneding explanations based on assumptions founded in sexist bias, those who declare it tend to assume the intentions and biases of the people they rail against for it.

I condescend to everyone, so I take it personal when someone says I am mansplaining. I'm not. I just think most people are fucking stupid so I have developed the tact of assuming I have to explain shit to everyone.
Furthermore, it is used primarily as a way to invalidate a point being made by a man, particularly in discussin feminine issues or when talking to a female.

So while the term may have had its origins in a legitimate phenomenon; sexist condescension on the part of men(btw, women mansplain too, I know plenty of women that assume men are dumb dogs); its colloquial use and thus its generally accepted connotation and reception is twofold.
For those that use it, it has become that men have no right to explain things to women.
For those that witness or are targeted by it, it is a tactic to shut down contradictory arguments made by a man to a woman that cannot be readily refuted using facts or evidence.