Frightful means causing fear or alarm, like a nightmare that jolts you awake or a sudden crash in the middle of the night. When something is frightful, it makes you feel scared or uneasy. A frightful storm might send you running inside, with lightning cracking across the sky and thunder shaking the windows.

The word can also describe something shockingly bad or unpleasant. If your room is in frightful condition, it's alarmingly disorganized, with clothes everywhere and books scattered across the floor. Someone might complain about a frightful headache or frightful traffic that made them hours late.

In older books, you'll sometimes see frightful used more playfully or dramatically than we use it today. A character might call something “frightfully expensive” or say they're “frightfully sorry,” using the word for emphasis rather than actual fear. But the core meaning stays the same: something frightful upsets you, whether it genuinely scares you or just strikes you as terribly wrong or unpleasant.