As wrong as two boys f--ckin'
Couldn't pour piss out of a boot with instructions written on the heel.
Wouldn't piss on him/her if they were on fire
On (top of) it like a pack of pit-bulls on a three legged cat.
I first saw a variation of that in <"The Girl, The Gold Watch and Everything" by John D. Mc Donald then it was: "She wouldn't piss in your mouth if your teeth were on fire".