Any exercise that has limited benefits for physical fitness, but boosts
one's sexual attractiveness or performance. The classic examples are bicep/forearm curls (for guys; these are purely for show), the hip abductor/adductor machines (for women; it's obviously translatable to how snugly you can squeeze
a guy's penis), and any sort of ab crunch thingy.
Don't fool yourself into thinking those hip adductors will make you lose weight; it's just a silly sexercise.