Yo mama's so thin she, got nipples on her back....
or
You sir are depriving a village somewhere of it's idiot.
or
difficult to believe you beat a 100 million other sperm
Slightly humerous story although you probably had to be there, when I was at college I got into a bit of Your mama rant on the hockey field. Afer making various remarks about the other Chaps mum, nan and sister (and a little 4 ball action), his mate turns round and says this guy's mum had died over the summer holiday. Cue extremely awkward silence, tumbleweeds blowing past with me left with only one thing to say. "No wonder she was so cr*p, if she was dead, should'nt he consider charging less for her next time round?"