Harebrained describes an idea or plan that's wildly impractical, reckless, or just plain foolish. The word combines hare (the fast, jittery animal) with brained, suggesting someone thinking as frantically and carelessly as a hare dashes around. A harebrained scheme might sound exciting at first but falls apart the moment you think it through.

When your friend suggests building a treehouse in one afternoon with no tools, no plan, and no experience, that's a harebrained idea. When a character in a story decides to sneak into a dragon's cave because “it'll probably be fine,” that's harebrained thinking.

The word carries a sense of both foolishness and recklessness. A harebrained plan is unlikely to work and makes sensible people shake their heads and ask, “What were they thinking?” Someone might launch a harebrained business venture without researching costs, or suggest a harebrained solution to a problem without considering the consequences.

Not every risky idea is harebrained, though. Some bold plans succeed despite long odds because they're backed by careful thought and preparation. Harebrained plans lack that crucial foundation: they're all enthusiasm and no common sense.