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late 14c., "find fault, lament," from stem of Old French complaindre "to lament" (12c.), from Vulgar Latin *complangere, originally "to beat the breast," from Latin com-, intensive prefix (see com-), + plangere "to strike, beat the breast" (see plague (n.)). Older sense of "lament" died out 17c. Related: Complained; complaining.