You almost never say this word alone. You put it after scare or frighten: to scare someone shitless is to make them extremely frightened — so frightened they can hardly think. I was scared shitless is the usual shape. It is rude, because of shit. You will hear it in informal talk and in films when someone wants to say they were not just scared, but terrified. Though actually the -less here does not mean “without shit” in a straight way. It is a dark joke that follows phrases like scare the shit out of someone: the fear is so strong it is as if something is shocked out of you. Do not use it with a teacher or in a shop. Terrified or really scared will do.