Someone who is unknowingly tricked into actively working against their side and themselves. The word comes from the game Among Us which is a murder mystery game where there are 2 murderers called imposters.
Yellow: I was in O2
Blue: No you weren’t, I saw you vent in electrical.
Green: yellows clear, if he were imposter he’d have killed me many times. Blue I think you’re the imposter.
Blue: Stfu green. You realize you’re the third imposter, right?
When you're just playing some good ol'e fashioned Among Us with the pals. You're relaxing having a great time then OUTTA NOWHERE Justien throws some sus on people. But he gets out of hand and really starts spreading the sus. But he's wrong and he's also not the imposter.