ply

Verb (1)

Middle English plien "to put on, around or over, use," aphetic form of aplien, applien "to join, combine, use for a certain purpose, apply"

Noun

borrowed from French pli "fold, pleat, crease," going back to Old French plei, pli "joint in armor, fold," noun derivative from pleier, plier "to fold, bend" — more at ply entry 3

Verb (2)

Middle English plien "to bend, fold, be capable of bending, be flexible, make submissive, be submissive," borrowed from Anglo-French pleier, plaier, ploier, plier "to fold, bend, (intransitive) bend, bow down, sag" (also continental Old French), going back to Latin plicāre "to fold, bend," back-formation from -plicāre in compounds such as applicāre "to bring into contact (with)," complicāre "to fold together," explicāre "to free from folds, straighten," implicāre "to fold about itself, entwine," replicāre "to fold back on itself," formed from a stem *-plec-, probably a variant, without the formative -t-, of the stem of plectere "to plait, twine," going back to Indo-European *pleḱ-t-, whence also Germanic *flehtan- "to braid, plait" (whence Old Saxon & Old High German flehtan "to plait, weave together," Old Icelandic flétta; in nominal derivation Old English flehta "wickerwork," flohtenfōte "web-footed," Gothic flahta "braid"), Old Church Slavic pletǫ, plesti "to weave (intrigues)"; from *pleḱ- alone in Greek plékein "to plait, braid, weave," with nominal derivatives in Greek plokḗ "weaving," plókos "braid, lock," Sanskrit praśna- "turban" and perhaps Avestan ərəzatō.frašna- "having a golden helmet/coat of mail"

Note: Latin plicāre developed regularly to Old French pleier, later ploier (Modern French ployer "to bend, bow"). The parallel medieval French verb plier (Modern French plier "to fold") arose by analogy with verbs such as prier "to ask," from Latin precārī, which had variants with -i- and -ei- depending on where stress fell on inflected forms, leading to generalization of one or the other form throughout the paradigm. In the case of pleier/ploier, the invasive form pli- first appeared under stress and later spread to unstressed forms. Older ploier was never displaced, however, and the maintenance of both forms lead to two independent verbs with slightly different senses. — In addition to forming the base of -plicāre in applicāre (see apply), complicāre (see complicate entry 2), explicāre (see explicate), implicāre (see implicate, imply), replicāre (see replicate entry 1, reply entry 1), the Latin stem *plec- may also be the second element of nominal compounds such as duplic-. duplex "folded double, having two parts" (see duplex entry 1), simplic-, simplex "consisting of one element" (see simple entry 1), etc. Ernout and Meillet (Dictionnaire étymologique de la langue latine) refer to -plicāre as an "intensive" ("un intensif") relative to plectere. Slavic *pletǫ (in the Old Church Slavic form cited above) must go back to *plek-t-, as the root with the palatovelar would result in an unattested *plestǫ. Alternatively, pletǫ could represent *ple-t- and be allied with Germanic *falþan- (see fold entry 1); this is the suggestion of Ernout and Meillet.