1. [ adjective ] afflicted with or characteristic of mental derangement

"was declared insane" "insane laughter"

Related terms: sane brainsick hebephrenic schizophrenic crackbrained paranoid maniacal crazed crazed psychopathic psychotic raving screw-loose lunatic mentally_ill maniclike non_compos_mentis fey balmy around_the_bend manic-depressive certifiable amuck irrational unreasonable
2. [ adjective ] very foolish
Synonyms: mad harebrained

"harebrained ideas" "took insane risks behind the wheel" "a completely mad scheme to build a bridge between two mountains"

Related terms: foolish
Similar spelling:   insanely