A person who is so stupid that any attempt to get them to understandcommon sense and/or decent human behavior is all but futile, and is as if you are talking to an actual ape. Almost always used in an aggressive or hostile way.
"Cassandra will fall in love with me one day, I know it for sure!"
"You've been saying that for over a year and she's made it very clear that she wants nothing to do with you. Get a grip, you fucking ape"