so senseless or unreasonable as to seem insane; crazy
crackbrained in American English
(ˈkrækˌbreind)
adjective
foolish, senseless, or insane
Word origin
[1625–35; crackbrain + -ed3]This word is first recorded in the period 1625–35. Other words that entered Englishat around the same time include: cabaret, concentration, deviate, etch, substratum-ed is a suffix forming adjectives from nouns. Other words that use the affix -ed include: bearded, monied, tender-hearted