late 14c., compleinen, "lament, bewail, grieve," also "find fault, express dissatisfaction, criticize," also "make a formal accusation or charge to an authority," from stem of Old French complaindre "to lament" (12c.), from Vulgar Latin *complangere, originally "to beat the breast," from Latin com-, here perhaps an intensive prefix (see com-), + plangere "to strike, beat the breast" (from PIE root *plak- (2) "to strike").
The sense evolution is from "expression of suffering" to "grievance; blame." The transitive sense of "lament" died out 17c. Also from late 14c. as "utter expressions of grief or pain," hence, chiefly poetically "emit a mournful sound" (1690s). Related: Complained; complaining.
"expression of suffering, grievance, blame," late 14c., verbal noun from complain (v.). Related: Complainingly.
early 15c., in law, "one who commences a legal action against another, one who makes a formal complaint in court," from Old French complaignant, present participle of complaindre (see complain). The present participle also was used as a noun in Middle French.
late 14c., "lamentation, expression of grief," also "grief, sorrow, anguish" itself; also "expression of dissatisfaction or disapproval; statement of grievances, formal accusation; a plaintive poem," from Old French complainte (12c.) "complaint, lament," noun use of fem. past participle of complaindre "to lament" (see complain). Meaning "that which is complained of" is from 1745; specific meaning "bodily ailment, cause of pain or uneasiness" is from 1705 (often in U.S. colloquial use generalized as complaints).
*plāk-, Proto-Indo-European root meaning "to strike."
It is the hypothetical source of/evidence for its existence is provided by: Greek plazein "to drive away," plēssein "to beat, strike;" Latin plangere "to strike, lament;" Old English flocan "to strike, beat;" Gothic flokan "to bewail;" German fluchen, Old Frisian floka "to curse."
"complaining, apt to complain," c. 1600, from Latin querimonia "a complaint," from queri "to complain" (see querulous).