Confirm

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Middle English: from Old French confermer, from Latin confirmare, from con- ‘together’ + firmare ‘strengthen’ (from firmus ‘firm’).

wiktionary

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From Middle English confirmen, confermen, from Old French confermer, from Latin confirmāre(“to make firm, strenghten, establish”), from com-(“together”) + firmare(“to make firm”), from firmus(“firm”).

etymonline

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confirm (v.)

mid-13c., confirmyn, confermen "to ratify, sanction, make valid by a legal act," 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 assimilated form of com"together," but here perhaps an intensive prefix (see con-), + firmare "to strengthen," from firmus "strong, steadfast" (from suffixed form of PIE root *dher- "to hold firmly, support").

From mid-14c. as "make firm or more firm, add strength to;" late 14c. as "make certain or sure, give an assurance of truth, verify." Related: Confirmative; confirmatory.