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.