late 13c., conforten "to cheer up, console, soothe when in grief or trouble," from Old French conforter "to comfort, to solace; to help, strengthen," from Late Latin confortare "to strengthen much" (used in Vulgate), from assimilated form of Latin com-, here probably an intensive prefix (see com-), + fortis "strong" (see fort). Change of -n- to -m- began in English 14c.
In Middle English also "give or add strength to" (c. 1300); "encourage, urge, exhort" (c. 1300). Related: Comforted; comforting.