In chess, a "poisoned" pawn is an unprotected pawn, typically one that has not yet moved from its starting position, that is currently under attack by an opponent's piece. It seems to be a free pawn to capture, but because of the nature of the position, its capture will lead the capturer into a positional disadvantage that will significantly reduce her chances of victory.
In chess theory, there are "poisoned pawn" variations of several well-known chess openings. The most famous of these is the poisoned pawn variation of the Najdorf Sicilian.