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mid-14c., from Middle French défectif (14c.) and directly from Late Latin defectivus, from defect-, past participle stem of deficere (see deficient). A euphemism for "mentally ill" from 1898 to c.1935. Related: Defectively; defectiveness.
defective de·fec·tive (dĭ-fěk'tĭv)
Having an imperfection or malformation.
Lacking or deficient in some physical or mental function.