The Flab Stab is the mistaken thrusting of your penis into the folds of flesh in a larger girl's thighs or buttocks rather than into her vagina. This typically only occurs at the moment of initial penetration, and hopefully only occurs when one is quite drunk.
"Dude, I'm embarrassed to say it but I took Brenda home last night and while I was trying to pork that fat slut I gave her the flab stab."
When someone is stabbed in a particularly meaty / fatty region of their body, it is noted as being a "Flab Stab". The buttocks, abdomen, chest, thighs and underarms are all places which contain a sufficiently high fat percentage to be regions where a "Flab Stab" is a possibility.
The knife was thrusted into Dr. Evil's buttocks at high speed, gushing blood and fat outwards. Dr. Evil slumped to the floor, motionless.
"Aha! So even the greatest fall from the Flab Stab!" said Mr. Raminov.