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.

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Definitions of ply from WordNet
ply (v.)
give what is desired or needed, especially support, food or sustenance;
Synonyms: provide / supply / cater
ply (v.)
apply oneself diligently;
ply (v.)
travel a route regularly;
Ships ply the waters near the coast
Synonyms: run
ply (v.)
join together as by twisting, weaving, or molding;
ply fabric
ply (v.)
wield vigorously;
ply an axe
ply (v.)
use diligently;
ply your wits!
ply (n.)
one of the strands twisted together to make yarn or rope or thread; often used in combination;
four-ply yarn
three-ply cord
ply (n.)
(usually in combinations) one of several layers of cloth or paper or wood as in plywood;