Mentally unbalanced, eccentric, or deranged; out of order or functioning badly.
From French détraqué, past participle of déraque (de- 'off' + traque 'track'). Borrowed into English in the 1800s, originally referring to something knocked off its rails or track, then used metaphorically for disordered minds.
Detraque is a delightfully French word that English borrowed to describe someone delightfully eccentric—think of a character so uniquely odd they seem mentally 'off the rails' in a charming way.
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