The curves of well-defined abdominal muscles that start at the lateral iliac crests and end medially at the pubic tubercle. They form the "V" that outlines a 6-pack. They are referred to as such both because they can be worked out during sexual activity, and possession of such musculature makes you want to have sex with that person.
Did you see his sex muscles when he took his shirt off? I think i just peed my pants a little.
A protusion of subcutaneous fat below the umbilicus and above the pubic area, often seen peeking out between ones pants and the bottom of a short shirt. It is an acronym for Fat Upper Pussy Area, although the term can be used to describe members of either gender.
Cindy's friend: "I wouldn't eat that donut Cindy, you should have your finger for dessert instead"
Cindy: "What? Why?"
Cindy's friend: "Look at that fu-pa. It must get cold hanging out of your shirt all day"