1: The abs that get you laid
2: Abs that are perfect for having sex
Example 1:
Hose: Dude after I got from boot camp I was totally ripped my girlfriend practically dragged me into the bed.
Gustavo: Really? What made her so horny?
Hose: My sex-pack abs
Example 2:
Bogs: Yo man I just had sex with my girlfriend for the first time with a six-pack. IT WAS AWESOME!
Matt: Duh man that's why they're called sex-pack abs