Green’s Dictionary of Slang

(US) to loaf, to idle.

1971		Playboy June 60: Man is going to need bigger and better weekend stimulation than fat-assing it in front of his TV set [HDAS].
1995–1997		FatAss Funtime Productions Fatass.com 🌐 Hi. I’m the fatass. This is my web site and this is my homepage. I got the nickname fatass from my college roomates who claim I was always fatassing around, whatever that means.