1 mentally ill/crazy
VERBS be, look | become, go He later became insane and was confined to an asylum. He went almost insane when he heard that his daughter had died. | drive sb You're driving me nearly insane with that noise. | certify sb, declare sb In 1975 she was certified clinically insane and sent to a mental hospital.
ADV. completely, totally | almost, nearly | a little | dangerously | clinically | criminally He is criminally insane, unable to stop himself attacking women.
2 very stupid/dangerous
VERBS be, seem It seems insane to cut the budget now.
ADV. quite, totally The whole idea is quite insane.