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]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "hare-brained":
   airy, asinine, brainless, crackpot, fanciful, flighty, foolhardy,
   foolish, frivolous, giddy, heedless, improvident, inane, madcap,
   mindless, rash, reckless, scatterbrained, silly, visionary, wild,
   witless



Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "harebrained":
   absurd, adventurous, arbitrary, audacious, balmy, bold, brainless,
   capricious, cranky, crazy, crotchety, daredevil, daring,
   death-defying, dizzy, empty-headed, fanciful, fantasied, fantastic,
   featherbrained, featherheaded, fire-eating, flaky, flighty,
   fluttery, foolhardy, forward, freakish, frivolous, gaga, giddy,
   giddy-brained, giddy-headed, giddy-pated, giddy-witted, harebrain,
   humorsome, insane, kinky, loony, madbrain, madbrained, madcap,
   maggoty, moody, motiveless, notional, petulant, potty,
   preposterous, presumptuous, quirky, rattlebrained, rattleheaded,
   rattlepated, scatterbrained, scramblebrained, shatterbrained,
   silly, skittish, temperamental, thoughtless, unreasonable,
   unrestrained, vagarious, vagrant, wacky, wanton, wayward,
   whimsical, wild, wild-ass, witless