In chess circles, a bad chess player. Often, but not always, an opponent who is not only much worse than you, but actively pisses you off with
poor sportsmanship, bad manners at the board (or online), or just general dumbassery.
Keep in mind, patzerism is relative. Everyone is a patzer to someone
better. A person who casually plays chess is a patzer to
even someone with a basic knowledge of tactics. That person is a patzer to a decent club player. The decent club player is a patzer to the club champion. The club champion is a patzer to
Alexandra Botez. Alex is a patzer to Levy Rozman. Levy is a patzer to even a mediocre
GM. Every other human being alive is a patzer to Magnus Carlsen, and Magnus is a patzer to Stockfish.