Scare cards could be any cards that come up on the community board that can help one player complete a possible draw or change a strong hand into a weak one. It’s a term often used by Texas Hold’em players.
For example, you’ve got an A, K suited in your hand. The flop brings up Q, J, 10, giving you a strong hand as you flop the nut straight. After the flop betting round, the turn brings an ace. After another betting round, the river brings another queen.
It’s obvious in this Q, J, 10, A, Q community board that the last queen is the scare card as your A, K hand is now behind several other hands, despite having a great starting hand.