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."
1610s, "a rope for hoisting," probably a blend or confusion of pendant in the nautical sense of "suspended rope" and pennon "long, narrow flag." Use for "flag on a warship" is by 1690s; as "flag long in the fly as compared with its hoist," 1815. The meaning "flag symbolizing a sports championship" (especially baseball) is from 1880; as a synonym for "championship" it is attested by 1915.
"long, narrow flag" (often triangular or swallow-tailed, attached to a lance and having distinguishing markings), late 14c., penoun, from Old French penon "feathers of an arrow; streamer, flag, banner," from penne "feather," from Latin penna "feather" (from PIE root *pet- "to rush, to fly"). In medieval Europe, the flag of a knight-bachelor or one who has not reached the dignity of a banneret.