The Collaborative International Dictionary of English v.0.48:

Ply \Ply\, v. i.
   1. To bend; to yield. [Obs.]
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            It would rather burst atwo than plye. --Chaucer.
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            The willow plied, and gave way to the gust.
                                                  --L'Estrange.
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   2. To act, go, or work diligently and steadily; especially,
      to do something by repeated actions; to go back and forth;
      as, a steamer plies between certain ports.
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            Ere half these authors be read (which will soon be
            with plying hard and daily).          --Milton.
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            He was forced to ply in the streets as a porter.
                                                  --Addison.
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            The heavy hammers and mallets plied.  --Longfellow.
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   3. (Naut.) To work to windward; to beat.
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The Collaborative International Dictionary of English v.0.48:

Ply \Ply\, v. t. [imp. & p. p. Plied; p. pr. & vb. n.
   Plying.] [OE. plien, F. plier to fold, to bend, fr. L.
   plicare; akin to Gr. ?, G. flechten. Cf. Apply, Complex,
   Display, Duplicity, Employ, Exploit, Implicate,
   Plait, Pliant, Flax.]
   1. To bend. [Obs.]
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            As men may warm wax with handes plie. --Chaucer.
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   2. To lay on closely, or in folds; to work upon steadily, or
      with repeated acts; to press upon; to urge importunately;
      as, to ply one with questions, with solicitations, or with
      drink.
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            And plies him with redoubled strokes  --Dryden.
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            He plies the duke at morning and at night. --Shak.
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   3. To employ diligently; to use steadily.
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            Go ply thy needle; meddle not.        --Shak.
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   4. To practice or perform with diligence; to work at.
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            Their bloody task, unwearied, still they ply.
                                                  --Waller.
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The Collaborative International Dictionary of English v.0.48:

Ply \Ply\, n. [Cf. F. pli, fr. plier. See Ply, v.]
   1. A fold; a plait; a turn or twist, as of a cord.
      --Arbuthnot.
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   2. Bent; turn; direction; bias.
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            The late learners can not so well take the ply.
                                                  --Bacon.
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            Boswell, and others of Goldsmith's contemporaries, .
            . . did not understand the secret plies of his
            character.                            --W. Irving.
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            The czar's mind had taken a strange ply, which it
            retained to the last.                 --Macaulay.
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   Note: Ply is used in composition to designate folds, or the
         number of webs interwoven; as, a three-ply carpet.
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WordNet (r) 3.0 (2006):

ply
    n 1: one of the strands twisted together to make yarn or rope or
         thread; often used in combination; "three-ply cord"; "four-
         ply yarn"
    2: (usually in combinations) one of several layers of cloth or
       paper or wood as in plywood
    v 1: give what is desired or needed, especially support, food or
         sustenance; "The hostess provided lunch for all the guests"
         [syn: provide, supply, ply, cater]
    2: apply oneself diligently; "Ply one's trade"
    3: travel a route regularly; "Ships ply the waters near the
       coast" [syn: ply, run]
    4: join together as by twisting, weaving, or molding; "ply
       fabric"
    5: wield vigorously; "ply an axe"
    6: use diligently; "ply your wits!"

The Free On-line Dictionary of Computing (19 January 2023):

ply

    1. Of a node in a tree, the number of
   branches between that node and the root.

   2. Of a tree, the maximum ply of any of its nodes.

   (1998-12-29)