(informal) To explain (something) condescendingly (to a female listener), especially to explain something the listener already knows, presuming that she has an inferior understanding of it because she is a woman.
2011 February 1, Linkins, Jason, “What's Behind The Drive To Redefine Rape In New And Insane Ways?”, The Huffington Post:
But what's getting all of the attention in the bill is the part where legislators have banded together to mansplain the various shadings of the crime of "rape" to America.
2012 May 18, Seltzer, Sarah, “Can Occupy Fight Back Against the War on Women?”, The Nation:
There were some Occupy-style solutions: those whose voices dominate should “step back” for an entire meeting. [...] Men should notice when they are “mansplaining” (this one got a thunderous ovation).
Positioning himself as the white guy mansplaining to a black woman what's best for her, The New York Time's John Caramanica argues that Rihanna's attempt, on the album, "to make public art with the person who physically abused you is immature, pre-feminist, post-ethics."