A short, casual word for an umpire — the official who stands on the field and decides what happened: was that ball inside the line, was the runner safe, is that a foul. You hear it most in baseball in America, and it belongs to the mouth, not the page. Fans in the stands shout it ("Come on, ump!"), players mutter it, and sports commentators use it when they are relaxed. A newspaper report would more likely write the full word umpire. Note that this word lives in the sports where people say umpire — baseball, tennis, cricket — and not in football or basketball, where the official is a referee (ref for short). So an ump is never in charge of a soccer match. It is not rude by itself, but because it is so often shouted in complaint, it can carry a small flavour of impatience.