A kook is someone who acts in strange, eccentric, or foolish ways that make others think they're a bit crazy or out of touch with reality. If your uncle insists that aliens built the pyramids and won't listen to any evidence otherwise, your parents might affectionately call him a kook. The word suggests someone whose ideas or behavior are odd enough to make people shake their heads, but not in a dangerous way.

In surfing culture, kook has a specific meaning: a beginner surfer who doesn't know proper surf etiquette or tries tricks they're not ready for, often getting in everyone's way and potentially causing accidents. Experienced surfers might call someone a kook for dropping in on another surfer's wave or for using equipment incorrectly.

The word carries a tone of gentle mockery rather than a serious insult. When you call someone a kook, you're saying they're harmlessly weird rather than genuinely threatening. A scientist with wild hair who talks to himself while working on equations might be considered a lovable kook. The term suggests that while someone's behavior might be unusual or misguided, they're mostly just entertainingly odd rather than actually harmful.