When no card can make you the winner.
Whatever you draw loses, so there is no value in calling. Example. If the opponent has a full house and you have a flush draw, you lose even if the flush comes in. It is the extreme case of dirty outs. In draw games the same happens when the opponent's pat hand is too strong: in 2-7, drawing one to 2-3-7-8 against a smooth pat hand of 9 or better is drawing dead in 45% of combinations.