"You are lower than a hog's belly."
"You are so low you have to look down to look up."
"You got no more sense than a boiled egg."
"If brains were dynamite, you wouldn't have enough to blow your nose."
"You nasty, yellow-bellied, sneakin', lyin', pestiliferous scorpion!"
"You monstracious, cockeyed, good-for-nothing, snaggletoothed gaub of fat!"
"You egg-suckin', sheep-stealin', toad-eatin', frog-hearted, flop-eared groundhog!"
"You filthy, turtle-backed, snake-headed, bowlegged ton and a half of soap grease!"
...Because calling people names should be fun!