Well, I'm not as young as Jazzo or BY, but I am young enough to know about phat. Phirst of all, it's already on the way out (off-the-hook or off-the-chain is a more up-to-the-second way to say "cool" -- it's a reference to meat, by the way). The AHD (yes, it's in there) says that the origin of "phat" is unknown, but notes that it used to mean "sexy". Apparently the word has been around in that sense since the 60s, and I have heard that it derives from the acronym Plenty of Hips And Thighs, but as most acronym-based etymologies are not true, I tend to doubt this one, too. I'll do some more research, though.