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late 14c., of diseases, "firmly established," past participle adjective from confirm. Of persons and their habits, from 1826.
mid-13c., confirmyn "to ratify," from Old French confermer (13c., Modern French confirmer) "strengthen, establish, consolidate; affirm by proof or evidence; anoint (a king)," from Latin confirmare "make firm, strengthen, establish," from com-, intensive prefix (see com-), + firmare "to strengthen," from firmus (see firm (adj.)). Related: Confirmative; confirmatory.