In terms of the National Football League the words “happy feet” are used to describe a quarterback who, when under pressure of being sacked, can’t seem to set his feet in the "pocket" and release the ball accurately. This might result in the ball being deflected or intercepted by the opposing defense.

It is generally not considered a good thing to be described as having “happy feet”.

In defense of the quarterback though, if I had a bunch of highly skilled 6’5’’ 300+ pound human beings bearing down on my ass intent on ripping my head off, you can bet your bottom dollar my feet would be more than happy to get me the hell out of there.