Rebecca Solnit's 2008 essay "Men who explain things" popularized the concept and the general awareness of this gentlemanly practice, but the word itself was not used therein. Instead, "Mansplain" was apparently first uttered on Livejournal a few weeks later by phosfate, a now-vanished psuedonymous user. This is revealed in Merriam-Webster's new official definition, crediting Know your Meme for the discovery. [via Kottke]
The splain of mansplain isn't new. It's been used to mean "explain" in texts showing informal speech or—as when Ricky Ricardo admonished his beloved in I Love Lucy—in imperfect English for at least a century. Since the advent and consequent blossoming of mansplain, splain has been attached to other morphemes to create words modeled on mansplain. Mother Jones put the word leftsplaining into the headline of an article by none other than Rebecca Solnit (about how people on the political left should stop expounding on the failings of lefties to fellow lefties). A politician under the mistaken impression that potlucks are a unique Iowa tradition was accused of potluck-splaining when he tried to illuminate non-Iowans on how the communal meals work.