Before "Phat" - there was just "Fat"
A dry Public Service employee who was also a keen fawlty towers fan, moved the meaning sideways a little from just being overweight, To a slightly sarcastic meaning of Having a good ol' time.
"You boys look like you're having a fat time"
a word used, mainly on stan twitter, to describe how someone acts, usually if they are being annoying or something. not used to describe someone’s weight.
user ivanschanel is fat