Etymology
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ply (v.1)

"work with, practice with persistence, use or employ diligently," late 14c., shortened form of applien "join to, apply" (see apply). The core of this is Latin plicare "to lay, fold, twist," from Proto-Italic *plekt-, from PIE root *plek- "to plait." The sense of "travel regularly, go back and forth over the same course" is attested from 1803, perhaps from earlier sense "steer a course" (1550s). Related: Plied; plies; plying.

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ply (n.)

"a layer, a fold," 1530s, from French pli "a fold" (13c.), alteration of Old French ploi "fold, pleat, layer" (12c.), verbal noun from ployer (later pleier) "to bend, to fold," from Latin plicare "to fold, lay" (from PIE root *plek- "to plait"). Often used to indicate the number of thicknesses of which anything is made; this also is the ply in plywood.

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ply (v.2)

"to bend, yield," late 14c., plien, from Old French plier, earlier pleier "to fold, bend," from Latin plicare "to lay, fold, twist" (from PIE root *plek- "to plait"). Related: Plied; plies; plying.

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multi-ply (adj.)

"having several layers or webs," 1887, from multi- "many" + ply (n.) "a layer, a fold."

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plier (n.)

"one who folds or plies," 1670s, agent noun from ply (v.).

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pliers (n.)

"small pincers with long jaws adapted for holding small articles," 1560s, plural agent noun from ply (v.2). French cognate plieur meant "folder."

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plywood (n.)

"board made of two or more thin layers of wood bonded together and arranged so that the grain of one runs at right angles to that of the next," 1907, from ply (n.) + wood (n.).

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plie (n.)

in ballet, 1892, from French plié, literally "bent," from plier "to bend," from Old French ploier "fold, pleat, layer" (12c.), verbal noun from ployer (later pleier) "to bend, to fold," from Latin plicare "to fold, lay" (from PIE root *plek- "to plait"). Compare ply (v.2).

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quadruplicate (v.)

"to make fourfold, double twice," 1660s, from Latin quadruplicatus, past participle of quadruplicare "make fourfold," from quadri- "four" (from PIE root *kwetwer- "four") + plicare "to fold" (see ply (v.1)).  The sense of "make or provide in four identical versions" is by 1879. Related: Quadruplicated; quadruplicating.

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*plek- 
Proto-Indo-European root meaning "to plait." It is an extended form of root *pel- (2) "to fold."

It forms all or part of: accomplice; application; apply; complex; complexion; complicate; complication; complicity; deploy; display; duplex; duplicate; duplicity; employ; explicate; explicit; exploit; flax; implex; implicate; implication; implicit; imply; multiply; perplex; perplexity; plait; plash (v.2) "to interlace;" pleat; -plex; plexus; pliable; pliant; plie; plight (n.1) "condition or state;" ply (v.1) "work with, use;" ply (v.2) "to bend; ply (n.) "a layer, fold;" replica; replicate; replication; reply; simplex; splay; triplicate.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prasna- "turban;" Greek plekein "to plait, braid, wind, twine," plektos "twisted;" Latin plicare "to lay, fold, twist," plectere (past participle plexus) "to plait, braid, intertwine;" Old Church Slavonic plesti "to braid, plait, twist," Russian plesti; Gothic flahta "braid;" Old Norse fletta, Old High German flehtan "to plait;" Old English fleax "cloth made with flax, linen."
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