It can mean one of two things :
1. When someone basically shoots themselves in the foot when doing something, screws themselves over, a self-fulfilling prophecy etc.
Or...
2. When you get beat at something or get your ass handed to you by someone in any way, shape or form.
( Note : Check yourself before you screw yourself. )
Simply put, don’t be a foot-ass. Lol.
Examples :
And to think, Tyler thought he was all that and a bag of Pretzels... Boy, what an absolute foot-ass!
Or...
Everything was fine until Phoebe started to shade us around her new friends... She’s been such a foot-ass lately...
Or...
Can’t you go 5 seconds without being a foot-ass?