To deliver a beat down (physically or verbally) to another (sorry) person in a way that makes the defeated party unable to walk with their chin up and chest out. Basically an ownage/pwn
Definition derived from the beat down that Cassius Clay (formerly known as Muhammad Ali) gave the now infamous Sonny Liston.
James: Dude don't make me come over an Cassius Clay you man!
John: Why don't you try it?
<James goes over to John and gives him a whooping>
Sean: John you just got Cassius Clayed fool!!