(exp.) Not of sound mind; not having the regular use of reason; hence, also, as a noun, an idiot; a lunatic; one devoid of reason, either by nature or from accident.

abnormal bereft of reason brainsick crackbrained cracked crazed crazy daft deluded demented deprived of reason deranged disoriented distraught flighty hallucinated insane irrational loco lunatic mad maddened manic mazed mental mentally deficient meshuggah moon-struck non compos not all there not right odd of unsound mind off psycho queer reasonless senseless sick stark-mad stark-staring mad strange tetched touched unbalanced unhinged unsane unsettled unsound wandering witless