also *plāk-, Proto-Indo-European root meaning "to be flat;" extension of root *pele- (2) "flat; to spread."

It forms all or part of: flag (n.2) "flat stone for paving;" flagstone; flake (n.) "thin flat piece,; flaw; floe; fluke (n.3) "flatfish;" placenta; plagal; plagiarism; plagio-; planchet; plank.

It is the hypothetical source of/evidence for its existence is provided by: Greek plakoeis "flat," plax "level surface, anything flat;" Lettish plakt "to become flat;" Old Norse flaga "layer of earth," Norwegian flag "open sea," Old English floh "piece of stone, fragment," Old High German fluoh "cliff."

"any rock which splits easily into flags," 1730, from flag (n.2) "flat, split stone" + stone (n.).

early 14c., "a flake" (of snow), also in Middle English "a spark of fire; a splinter," from Old Norse flaga "stone slab, layer of stone" (see flag (n.2)), perhaps used here in an extended sense. Old English had floh stanes, but the Middle English form suggests a Scandinavian origin. "The close resemblance in sense between flaw and flake is noteworthy" [OED]. Sense of "defect, fault" first recorded 1580s, first of character, later (c. 1600) of material things; probably via notion of a "fragment" broken off.

British slang for "cigarette" (originally, especially, the butt of a smoked cigarette), 1888, probably from fag "loose piece, last remnant of cloth" (late 14c., as in fag-end "extreme end, loose piece," 1610s), which perhaps is related to fag (v.), which could make it a variant of flag (v.).

"to droop, decline in strength, become weary" (intransitive), 1520s, of uncertain origin; OED is content with the "common view" that it is an alteration of flag (v.) in its sense of "droop, go limp." Transitive sense of "to make (someone or something) fatigued, tire by labor" is first attested 1826. Related: Fagged; fagging.

also flag-man, "signaler," 1832, from flag (n.1) + man (n.). Earlier it meant "admiral" (1660s).

also flag-pole, 1782, from flag (n.1) + pole (n.1). Flagpole-sitting as a craze is attested from 1927.

also flag-ship, 1670s, a warship bearing the flag of an admiral, vice-admiral, or rear-admiral, from flag (n.) + ship (n.). Properly, at sea, a flag is the banner by which an admiral is distinguished from the other ships in his squadron, other banners being ensigns, pendants, standards, etc. Figurative use by 1933.

1610s, from flag (n.) + staff (n.). The settlement in Arizona, U.S., said to have been so called for a July 4, 1876, celebration in which a large flag was flown from a tall tree.

"a sign of danger or warning," 1777, from red (adj.1) + flag (n.1). A red flag was used as a symbol of defiance in battle on land or sea from c. 1600.