late 14c., "a smithy," from Old French forge "forge, smithy" (12c.), earlier faverge, from Latin fabrica "workshop, smith's shop," hence also "a trade, an industry;" also "a skillful production, a crafty device," from faber (genitive fabri) "workman in hard materials, smith" (see fabric). As the heating apparatus itself (a furnace fitted with a bellows), from late 15c. Forge-water (1725), in which heated iron has been dipped, was used popularly as a medicine in 18c.
early 14c., "to counterfeit" (a letter, document, etc.), from Old French forgier "to forge, work (metal); shape, fashion; build, construct; falsify" (12c., Modern French forger), from Latin fabricari "to frame, construct, build," from fabrica "workshop" (see forge (n.)). Meaning "to counterfeit" (a letter, document, or other writing) is from early 14c.; literal meaning "to form (something) by heating in a forge and hammering" is from late 14c. in English, also used in Middle English of the minting of coins, so that it once meant "issue good money" but came to mean "issue spurious (paper) money." Related: Forged; forging.