Insane, mad, crazy, crazed, demented, deranged, lunatic, maniac, non compos mentis are comparable in their general or nontechnical senses (for senses of corresponding nouns used technically see INSANITY ) and as meaning afflicted by or manifesting unsoundness of mind or an inability to control one’s rational processes.
Insane as applied to persons usually implies such unsoundness of mind that one is unable to function safely and competently in ordinary human relations, usually does not recognize one’s own condition, and is not responsible for one’s actions. In more general use insane implies utter folly or irrationality; the person or the act or utterance so described is, by implication, governed by blind passion or senselessness.
Mad usually implies more frenzy than insane and therefore carries a stronger suggestion of wildness, rabidness, raving, or complete loss of self-control.
Crazy often suggests such mental breakdown as may result from illness or old age or it may suggest a distraught or wild state of mind induced by some intense emotion (as anxiety, grief, joy, desire, or excitement).
As applied to such things as schemes, projects, or notions crazy usually suggests that they are the product of a disordered or ill-balanced mind.
Crazed is often used in place of crazy when a temporary disorder, usually with a specific cause, is implied.
Demented and deranged are more formal than the preceding words and less rich in connotations; both terms, moreover, imply a change from mental soundness to unsoundness, demented usually suggesting clear signs (as profound apathy or incoherence in thought, speech, or action) which indicate deterioration of the mental powers and deranged (compare derangement under ABERRATION 2 ) suggesting a loss of mental balance or a state of mental disorder resulting from a functional disturbance of the brain or nervous system.
Lunatic is approximately the equivalent of insane but is less frequently applied to persons and may imply no more than extreme folly <consuming with lunatic speed the assets of the earth —Agar > Maniac comes closer to mad, for it commonly connotes violence, fury, or raving.
Non compos mentis (Latin for “not sound of mind”) is a legal term which specifies a state, but does not define the particular condition or kind, of mental unsoundness. It is often used, especially in its shortened form non compos , more generally with similar indefiniteness.