It’s an adjective and it means having or showing a bad temper; being annoyed and irritable; being cranky or disagreeable. You know the sort of person, they come into work pissed off with everything and everybody and just looking for an excuse to blame somebody for everything that is wrong in their crappy lives, the world, the universe. Thoroughly unpleasant shitbags.
When a one up just isn't good enough and some "icing on the cake" is deemed necessary. Occasionally, the topper is topped without rebuttal. Both the "One Up" and the "Topper" are unusually suspicious.