Ply
late Middle English (in the sense ‘fold’): from French pli ‘fold’, from the verb plier, from Latin plicare ‘to fold’.
wiktionary
From Middle English pleit, plit, plite(“a fold, pleat, wrinkle; braid, strand in a braided cord, ply”), from Anglo-Norman pli, plei, pleit, [1] and Middle French pli, ploy, ply(“a fold, pleat; joint in armour; situation, state”) (modern French pli(“a fold, pleat”)), from plier, ployer(“to bend, fold”), [2] from Latin plicāre, present active infinitive of plicō(“to bend, fold, roll up”), from Proto-Indo-European *pleḱ-(“to fold, plait, weave”).
From Middle English plīen, pli, plie(“to bend, fold, mould, shape; to be flexible; to be submissive, humble oneself; to compel someone to submit”), [3] from Anglo-Norman plier, plaier, pleier, ploier, and Middle French plier, ployer(“to bend, fold; to be submissive; to compel someone to submit”) (modern French plier, ployer), [4] from Old French ploiier, pleier(“to fold”), [3] from Latin plicāre(“to fold”); see further at etymology 1. The word is cognate with Catalan plegar(“to bend, fold”), Italian piegare(“to bend, fold, fold up”), Old Occitan plegar, plejar, pleyar(“to fold”) (modern Occitan plegar), Spanish plegar(“to fold”). [4]
From apply; [5] compare Middle English plīen, pli, plie, pleie(“to place (something) around, on, or over, to cover; to apply, use; to strive”), [6] short for aplīen, applīen(“to combine, join; to attach; to assemble; to use, be of use; to allot; to apply; to inflict; to go; to ply, steer; to comply, submit”), from Old French applier, aplier, aploier(“to bend; to apply”), [7] from Latin applicāre, present active infinitive of applicō(“to apply; to attach, join; to add”), from ad-(“prefix meaning ‘to, towards’”) + plicō(“to bend, fold, roll up”); see further at etymology 1.
etymonline
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.
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.
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.
