Thu-Huong Ha, for Quartz:
The team of lexicographers charged with adding new words to the historical dictionary recorded the first documented use of “mansplain” in August 2008. In an exchange between two bloggers on Livejournal, electricwitch, a then-22-year-old fan of Bowie and Bollywood based in the Netherlands, accused count-vronsky, who has since deactivated their account, of mansplaining.
I think I first heard this term on Twitter, quite recently, probably near the end of 2017. A guy — naturally — was being an ass towards a woman. He continued being even more of an ass, when someone accused him of “mansplaining”.