Ghastly means shockingly horrible or frightening, like something that makes you gasp or recoil. When you see a ghastly accident scene in a movie, it's disturbing enough to make you look away. A ghastly mistake might be one so bad it makes everyone cringe.

You might describe someone's face as looking ghastly when they're terribly sick or frightened, pale and drawn in a way that's unsettling to see. A ghastly scream pierces the night. A ghastly discovery in a mystery novel makes the detective's blood run cold.

People also use ghastly more casually to mean “extremely bad” or “awful,” especially in British English. Someone might call a sweater ghastly if it's hideously ugly, or describe a boring lecture as ghastly. But even in these lighter uses, the word keeps its edge: it means worse than just bad, something that provokes a strong negative reaction. Whether you're describing something truly terrifying or just memorably terrible, ghastly suggests the kind of awfulness you can't ignore or forget.