The Oxford Thesaurus An A-Z Dictionary of Synonyms - hare-brainedadj.

1) rash, foolhardy, wild, madcap, reckless, heedless, improvident, visionary, fanciful, airy, Colloq crackpot: Don't put money into any of Zenobia's hare-brained schemes. 2) foolish, silly, inane, asinine, flighty, witless, brainless, mindless, giddy, frivolous, scatterbrained: Such hare-brained behaviour is not expected in two sober adults.