Definition of scary card
In French, a "scary card" would be translated as "une carte qui fait peur". A scary card is a card that overdrives a board. If a flop is 8-9-T with 2 spades and the turn is a J of spades, the J of spades is a scary card. In other words, on this card, all players should be careful because the hand possibilities are enormous.
Let's take a flop like 7-T-J with two spades. Here, the board offers several draws. Now let's imagine a turn that would be a "scray card" and turn this dynamic flop into a scary board: an 8 of spades. With the 8 of spades, our opponent could have several 2 pair combinations, he could have several straights, he could have a flush. A scary card is a card that makes the board scary, i.e. a board where the possibilities are enormous. On the one hand, when a player bets on a scary board or a scary card, you have to give him credit. But on the other hand, a scary board or scary card is an excellent candidate for a bluff. If you bluff a scary card and your opponent has nothing, he will often give you credit and fold.