unsporting
Not fair or respectful when playing a game or sport.
Unsporting means behaving in a way that goes against the spirit of fair play and respect that games and competitions are supposed to have. When someone does something unsporting, they might be following the technical rules but violating the unwritten code of honor that makes competition meaningful.
An unsporting player might mock an opponent who makes a mistake, deliberately try to injure someone in a contact sport, or take advantage of an opponent's misfortune in a way that feels wrong. For example, if your opponent in tennis trips and falls, pausing to let them recover would be sporting behavior. Immediately hitting a winner while they're still down would be unsporting.
The word captures something important: you can win a game and still lose something more valuable by being unsporting. When a soccer player fakes an injury to get the referee to penalize the other team, they might gain an advantage, but they've acted in an unsporting way that cheapens their own victory.
Being unsporting is different from simply being competitive or trying hard to win. It means crossing the line into behavior that makes the game less fair or honorable. People remember unsporting behavior long after they forget who won the actual game.