Demented

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mid 17th century: past participle of earlier dement ‘drive mad’, from Old French dementer or late Latin dementare, from demens ‘out of one's mind’.

Past participle of dement, from Latin dēmentāre.

demented (adj.)

"having lost the normal use of reason, afflicted with dementia," 1640s, from obsolete dement "drive mad." Related: Dementedness.