ur mom
What people say when the they lack the intelligence to think of any sort of comeback. It's a very redundant phrase. You'll say, "Hey, how's it goin'?" and your mother will be brought into the conversation out of nowhere.

Whomever uses this phrase in an argument, has automatically lost.
Me:Seriously, Napoleon Dynamite sucks.
Them:ur mom
Them:lol
Them:...
Me: Ohhh, original.
Them:STFU ur momz a ho
Me: At least I got one, motherfucka'!
Them has signed off