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.