Etymology
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complain (v.)

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.

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uncomplaining (adj.)
1744, from un- (1) "not" + past participle of complain (v.).
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complaining (n.)

"expression of suffering, grievance, blame," late 14c., verbal noun from complain (v.). Related: Complainingly.

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complainer (n.)

mid-15c., in law, "one who brings suit" (a sense now in complainant), agent noun from complain (v.). From 1520s as "a fault-finder, a grumbler."

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complainant (n.)

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.

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complaint (n.)

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).

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*plak- (2)

*plāk-, Proto-Indo-European root meaning "to strike."

It forms all or part of: apoplexy; cataplexy; complain; fling; paraplegia; plaint; plangent; plankton; planxty; plague; plectrum; quadriplegia.

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."

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querimonious (adj.)

"complaining, apt to complain," c. 1600, from Latin querimonia "a complaint," from queri "to complain" (see querulous).

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grudge (v.)
mid-15c., "to murmur, complain," variant of grutch. Meaning "to begrudge, envy, wish to deprive of" is c. 1500. Related: Grudged; grudges; grudging; grudgingly.
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grouse (v.)
"complain," 1885 (implied in agent noun grouser), British Army slang, of uncertain origin. OED notes "a curious resemblance" to Normandy French dialectal groucer, from Old French groucier, grocier "to murmur, grumble, complain," which is of imitative origin (compare Greek gru "a grunt," gruzein "to grumble;" also see grutch). Related: Groused; grousing. As a noun from 1918, from the verb.
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