*dher-

Middle English affermen, affirmen, "to decide upon" (c. 1300); "to state positively" (late 14c.), from Old French afermer (Modern French affirmer) "affirm, confirm; strengthen, consolidate," from Latin affirmare "to make steady, strengthen," figuratively "confirm, corroborate," from ad "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (from suffixed form of PIE root *dher- "to hold firmly, support").

The spelling was refashioned 16c. in French and English on the Latin model. The legal sense of "declare solemnly (as before a court) but without an oath" is from early 15c. Related: Affirmed; affirming.

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.