plyyoudaoicibaDictYouDict[ply 词源字典]
ply: English has two distinct words ply, although ultimately they are related. The one meaning ‘fold, twist, layer’ [14], now mainly found in plywood [20] and in combinations such as twoply and three-ply, comes from Old French pli, a derivative of the verb plier ‘bend, fold’ (source of English apply [14], pliable [15], pliant [14], pliers [16], and reply).

This went back to Latin plicāre ‘fold’, a relative of English fold and source of accomplice, complicate [17], employ, explicit, imply, pleat, plight ‘predicament’, and supplicate. It was formed from a base that also produced English perplex [16] and the final syllables of simple and supple. The apple pie of apple-pie bed [18] is thought to be an alteration of French nappe pliée ‘folded sheet’. Ply ‘travel a route regularly’ or ‘solicit’ (as in ‘ply for hire’) [14] is short for apply, a relative of ply ‘fold’, and originally meant ‘apply, employ’ (as in ‘ply one’s needle’).
=> accomplice, apply, complicate, comply, double, employ, explicit, fold, imply, perplex, pleat, pliable, pliers, plight, reply, simple, supple, supplicate[ply etymology, ply origin, 英语词源]
ply (v.1)youdaoicibaDictYouDict
"work with, use," late 14c., shortened form of applien "join to, apply" (see apply). The core of this is Latin plicare "to lay, fold, twist," from PIE root *plek- "to plait, twist" (cognates: Greek plekein "to plait, twine," plektos "twisted;" Latin plectere (past participle plexus) "to plait, braid, intertwine;" Old Church Slavonic plesti "to braid, plait, twist;" Gothic flahta "braid;" Old English fleax "cloth made with flax, linen").

Sense of "travel regularly" is first 1803, perhaps from earlier sense "steer a course" (1550s). Related: Plied; plies; plying.
ply (n.)youdaoicibaDictYouDict
"a layer, a fold" 1530s, from Middle 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" see ply (v.1)). This is the ply in plywood.
ply (v.2)youdaoicibaDictYouDict
"to bend," late 14c., plien, from Old French plier, earlier pleier "to fold, bend," from Latin plicare "to lay, fold, twist" (see ply (v.1)). Related: Plied; plies; plying.