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

1817, first used by Arctic explorers, probably from Norwegian flo "layer, slab," from Old Norse flo, from Proto-Germanic *floho-, from PIE root *plak- (1) "to be flat." Related to first element in flagstone. Earlier explorers used flake. Floe-rat was a seal-hunter's name for the ringed seal (1862).

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*plak- (1)

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."

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