“You’re about as sharp as a sack of wet mice.”
“You’re about as useful as tits on a boar hog.”
“You’re more full of shit than a Tiajuana circus elephant.”
“You’re more fucked up than a soup sandwich.”
“You couldn’t find your ass with both hands and a flashlight.”
“You’re like 10 pounds of cat shit in a 5 pound sack. A real mess.”
When I am lending a hand to someone who is really screwing something up, and they look to me for a more help than I am feeling like giving them:
Me: “Hey, you’re the one fucking this chicken, I’m just holding it for you!”
I don’t get this one. Could you please explain?
Wizard of Oz reference. The scarecrow was hoping to get a brain. So, you better get in front of him in case they only have one brain to give. Because you need a new brain.
What was the scarecrow asking the Wizard for? A brain.
“I better punch my give-a-shit clock.”
Sometimes I borrow Elaine’s drippingly sarcastic, “Ooh, well-crafted.”
It’s more subtle than “Your mother sucks dicks in hell.”
Keillor’s Guy Noir said, “I’m glad your mother isn’t here. She’d be very disappointed in you.”
Yeah? Well, your mama’s boots have instructions written on the heel.
(Holding up two fingers) Two boxes of rocks.
(After some fool has threatened to kick your ass) “Imagine that!” or “Fantastic!” They both mean “You’re dreaming.”
“And your point is…?”
Oh yeah. Been on the receiving end of that one. Once. It’s all it takes to form a scar.
On the same road trip, we were in two vans and communicating by CB (Pre-cell-phone-commonality days). In the other van was a man we called ‘Man Mountain’. Guy was hugely muscled. Anyway, as he’s in the other van, I feel safe taking pot-shots at him, in jest. I made some remark about how low his IQ was, and got this:
“ArrMatey! , why do you continue to shoot at my intelligence?”
I shrugged to those in the van with me, clicked the send button, and replied, “I like a small target?”
My favorite comment regarding idiot drivers: “Why do you buy another box of Cracker Jacks and try for a pilot’s license this time?”
Heehee!
I have heard a variation of this somewhere, when someone is being helped by someone who’s really not that much help: “Who’s fucking this goat anyway?”
I don’t do comebacks. I can never think of anything. My all-time favorite, though, is one that was done to me. I was arguing with a friend, and I meant to say, “I’m going to kick your ass!” But I misspoke, and said “kiss” instead of “kick.” (that’s another reason I don’t do comebacks; I misspeak a lot) He didn’t say anything for a moment and then said, deadpan, “I’m not into that.”
Over ten years ago, after the much larger girl who threatened to kick my ass for a perceived pass on my part towards her fugly boyfriend:
Storm: (standing up from her seat, looking Big McBitchy straight in the eye and speaking in a very calm, normal tone of voice) “Come and get me, bitch, if you think you can.”
:eek:
On any other day on which I *wasn’t * insane, I’d have told the girl no, I wasn’t groping your (gak!) boyfriend, sorry if you thought I was, can we all just get along?
But I was in a tremendously bad mood which had juuuust started to improve by imbibing a couple of nice Everclear margaritas with some friends. Then Big McBitchy came along past our table and started ranting about how I’d grabbed her boyfriend’s ass as we had walked by going to our table.
“Bwa?” said I. “I think you’ve got the wrong person, I did not grab anyone’s ass. I think you might be mistaken.”
Big McBitchy, sloppily drunk: “Yes, you did! I saw you do it!” Meanwhile, the boyfriend is shaking McBitchy’s shoulder, telling her to stop it, that no one grabbed his ass, what the hell is she talking about?
“You did do it and you’re a liar! I should kick your ass right here and now!”
At which point I stood up and delivered my comeback. I think I looked just a tad unhinged at that point and she backed off with a little help from Fugster the Boyfriend.
“I catch you trying to grab him again and I’m not gonna back off next time!”
Storm: “Okay, you do that.”
Don’t know where I got the (ahem) cajones, but it worked.
Oh, yeah. It was the alcohol. 
I was hanging out with some friends after work, we’d had a few, and somehow we got to talking about how many sexual partners we’d had. I turned to one of the guys and asked, “So how many women have you been with?”
He said, probably counting in his head, “How many women have I been with?”
I snapped back, “Well, you can include the boys and the goats too, if you want to.” Next thing I know the other guy had fallen on the floor, screaming like a banshee. He couldn’t believe little demure ivylass had said such a thing. 
I have always liked and ofetn used:
When I want your opinion I’ll beat it outta ya.
and for whiners:
Well, I guess there’s nothing left but to kill yourself.
A friend of mine has recently taken to responding to converstaions he wishes to end:
Smoke Crack…Worship Satan!
It typically brings unwanted discussion to an abrupt halt.
A guy at work used to proclaim
“I’ve been married three times to two women” to which the other guy said softly “What was the third one?”
It took me years to realize the double joke in this, that if the instructions were on the heel, the piss would pour out on its own.