Definition of scary board
A "scary board" is a scary picture. It is generally understood to mean that the painting offers several prints and possibilities. If the painting is A-J-T-9 with four spades, it is a scary board. On a scary board, players will usually be cautious. Here, it is possible that one or more players may have a straight, two pairs, a flush, three of a kind, etc. A scary board is all the more frightening when there are several players in the hand.
An scary card can turn a board into a scary board
Let's take a flop like 7-T-J with two spades. Here, the board offers several prints. Now let's imagine a turn that would be an "scary card" and that would transform this dynamic flop into a scary board: an 8 of spades. With the 8 of spades, our opponent could have several combinations of 2 pairs, he could have several straights, he could have one suit. An scary card is a card that makes a board scary, i.e. a board on which the possibilities are enormous. On the one hand, when a player bets on a scary board or an scary card, he must be given credit. But on the other hand, a scary board or scary card is an excellent candidate for bluffing. If you bluff an scary card and your opponent has nothing, he will often give you credit and fold.