Negative freeroll is a colloquialism when the only two outcomes of a situation are neutral, or negative; contrasted with a freeroll, where the only two outcomes are neutral or positive.
The phrase is most commonly used in poker, when a player places an all-in wager on a hand that can only either tie or lose. A negative freeroll is relatively common in low stakes amateur play, but in higher stakes play, it is considered a serious blunder to allow a negative freeroll on the final round of betting due to the amount of risk involved.