The defined muscle of a toned man or a toned woman, which runs from the hip down to the crotch region (obviously more prominent and commonly seen on men, haha!). Very sexy to see on men who are ripped, or just in general!
"omg! did you see that guy with EPIC hippage!?...so sexy!"
"gotta love a topless man with epic hippage!"
An example as used in another term of the same meaning: Fight Club Lines: Brad Pitt with his shirt off in Fight Club. Or Jared Leto (of 30 Seconds to Mars/Requiem for a Dream fame) in Fight Club.