So this guy goes to the zoo, and at the gorilla enclosure he notices this huge silverback copying his movements. So he waves his arms over his head. The gorilla waves his arms over his head. The guy does a “monkey tickle” of his armpits. The gorilla does, too. The guy jumps up and down. The gorilla copies him. The guy pulls one eyelid down… and the gorilla leaps out of the enclosure and pummels the guy into a steaming pile.
In the hospital, the zoo officials question the guy. He describes each thing he did until he gets to the eyelid gesture. “You idiot!” the zookeeper says, “Don’t you know that means ‘fuck you’ in gorilla?”
When the guy heals, he goes back to the zoo and throws a Louisville Slugger in the gorilla pen. He jumps up and down. The gorilla jumps up and down. He tickles his armpits. The gorilla does the same. The guy picks up a nerf bat and smashes himself in the face a dozen times.
The gorilla picks up the bat, looks at the guy, and pulls down his eyelid.
Best explanation I have, but then, I really hate THE.