The Collaborative International Dictionary of English v.0.48:
Harebrained \Hare"brained`\ (h[^a]r"br[=a]nd`), a. Wild; giddy; volatile; heedless. "A mad hare-brained fellow." --North (Plutarch). [Written also hairbrained.] [1913 Webster]WordNet (r) 3.0 (2006):
harebrained adj 1: very foolish; "harebrained ideas"; "took insane risks behind the wheel"; "a completely mad scheme to build a bridge between two mountains" [syn: harebrained, insane, mad]