harebrain (?.)

• birdbrain
• crackbrain
• crank
• cuckoo
• ding-a-ling
• featherbrain
• featherhead
• flibbertigibbet
• giddybrain
• giddyhead
• kook
• lunatic
• nut
• rattlebrain
• rattlehead
• rattlepate
• scatterbrain
• scatterbrains
• screwball
• shallowbrain
• shallowpate
• shatterbrain