Essentially, flipping the hippo is a clean term that refers to sexual activities. The term is used as a verb phrase, and is often used to discretely discuss the weekend's activities without snoopers finding out your intentions.
(Friend to Friend, with others nearby) - We went out to dinner, and then that night we were flipping the hippo until she had to go back home.
to make love raucously; to engage in sexual intercourse
Dude! Did you and Sasha flip the hippo last night? You were gone for hours!
Sorry I couldn't hang out yesterday, I was really sore from flipping the hippo with O'Glory.
I was thinking we could do something special tonight... Maybe some white wine, some turkey, we could flip the hippo--what do you say Willis?