Sinister means threatening or suggesting that something evil or harmful might happen. When something feels sinister, it creates an uneasy sense that danger lurks nearby. A sinister laugh in a dark hallway, a sinister figure watching from the shadows, or a sinister plot to sabotage someone's work all carry this quality of hidden menace.

The word captures threatening behavior that suggests intention, a deliberate plan to cause harm. A creaky old house might be spooky, but it becomes sinister when someone inside means you harm. When a villain in a story has sinister motives, they're planning something bad with purpose and cunning.

You might describe a sinister smile on someone's face when they're plotting mischief, or talk about the sinister implications of finding mysterious footprints outside your tent at night. The word adds a layer of foreboding, like background music in a movie that tells you something bad is about to happen.