Dad was a funny guy. He’s the one who introduced me to that phrase. But what, exactly, does it mean?
[ul][li]That the target has no parents, and that he’s been fooled into thinking his parents were really his? (That one would be pretty harsh if the guy was adopted.)[/li]That the target is brain-dead?[/ul]
The latter is my guess. At least that is the way I use it.
I always thought it was #2. I also always thought that it was a pretty poor joke because it doesn’t make much sense. “Dead” isn’t an insult you hear people flinging around.
I always thought #2.
snort You said ‘number twos’!
Which seems to support that your parents did not. 
Given that one and the Full Metal Jacket comment of:
“The best part of you slid down the crack of your mamma’s ass and ended up as a brown stain on the mattress! I think you been cheated!”
I’d be willing to bet it’s a suggestion that the target is not in fact a living breathing human, but simply a disgusting, lifeless, thoughtless lump of human byproduct tissue. In other words, a vaginally delivered mass of tissue in no other way resembling a human being. And Drill oughtta know!
See also Drill’s other terms of endearment:
Maggot, Crack-baby, Scumbag, Son of a motherless goat, Slimey (little) F***, Worm, Gellatinous mass, (Brain) tumor, Piece of S***, etc.
A variation of this that I use when appropriate is “Did your mom have any sons?”
A couple years ago, I stopped in a liquor store, bus kid in tow. Picked her up some Snapple, and myself a 12 of Harp. Went to the counter, and the kid there looked at the Harp, made a face and said something like “hoo-boy, that looks pretty out there, me I’m a Silver Bullet man myself.”
To which I replied…
Wow, you found someone proud to drink Coors. Usually if someone offers a Coors or Coors Lite it is with some form of Apology. Even my FIL who likes Coors, apologizes when he is offering it to others.
Jim
It was Iowa. That’s probably all the explaining needed.
I’ve always used it to mean “You’re so stupid you aren’t really a human being”, which fits in with the meanings **Inigo Montoya ** lists. My voice drips sarcasm when I say it.