A quick, wild, lazy, or perhaps random shot from a gun where you really don't expect to hit the person but are shooting in their general direction anyway.
Here is it used in a sentence: "He took pot shots at them from far away with a pistol."
Now obviously your chances of hitting things at extreme range with a pistol are very low. Thus it probably qualifies as a "pot shot."
I think the idea is to either make them keep their head down and be wary or to just take a whole lot of pot shots in the hope that one of them will hit. I have no clue as to the actual origin, I've just heard it used in context. I think I first read it in a Hardy Boys novel.

Pot shot. n.

Lit., a shot fired simply to fill the pot; hence, a shot fired at an animal or person when at rest or within easy range, or fired simply to kill, without reference to the rules of sport; a shot needling no special skill.

 

© Webster 1913.