A hippo that has ceased to be hip.
Word used to describe anything that thinks it's cool, but isn't or anything that was once cool, but isn't anymore due to arrogance or a changing of the times.
Pachelbel the Panther: See Herbie over there? Think he all fly 'n shit, yawning like he don't give a fuck 'bout whether he gets mauled by Leo. He one straight opotamus.
Andy the Aardvark: Yeah, man. Think he all badass 'n cool, when he really just a bigass, lazy mo'fucka.
Hungry Hippos are the opposite of opotamus.
Bell bottoms are opotamus.