What does "hairbrained" mean?

Hairbrained: See Harebrained. HAIRBREADTH; HAIR'SBREADTH Hair"breadth`, Hair's"breadth`, a. The diameter or breadth of a hair; a very small distance; sometimes, definitely, the forty-eighth part of an inch. Every one could sling stones at an hairbreadth and not miss. Judg. xx. 16

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