Source : Webster's Revised Unabridged Dictionary (1913)

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.]

            As men may warm wax with handes plie. --Chaucer.

   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.

            And plies him with redoubled strokes  --Dryden.

            He plies the duke at morning and at night. --Shak.

   3. To employ diligently; to use steadily.

            Go ply thy needle; meddle not.        --Shak.

   4. To practice or perform with diligence; to work at.

            Their bloody task, unwearied, still they ply.
                                                  --Waller.

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.

   2. Bent; turn; direction; bias.

            The late learners can not so well take the ply.
                                                  --Bacon.

            Boswell, and others of Goldsmith's contemporaries, .
            . . did not understand the secret plies of his
            character.                            --W. Irving.

            The czar's mind had taken a strange ply, which it
            retained to the last.                 --Macaulay.

   Note: Ply is used in composition to designate folds, or the
         number of webs interwoven; as, a three-ply carpet.

Ply \Ply\, v. i.
   1. To bend; to yield. [Obs.]

            It would rather burst atwo than plye. --Chaucer.

            The willow plied, and gave way to the gust.
                                                  --L'Estrange.

   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.

            Ere half these authors be read (which will soon be
            with plying hard and daily).          --Milton.

            He was forced to ply in the streets as a porter.
                                                  --Addison.

            The heavy hammers and mallets plied.  --Longfellow.

   3. (Naut.) To work to windward; to beat.

Source : WordNet®

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: provide what is desired or needed, especially support, food
          or sustenance; "The hostess provided lunch for all the
          guests" [syn: {provide}, {supply}, {cater}]
     2: apply oneself diligently; "Ply one's trade"
     3: travel a route regularly; "Ships ply the waters near the
        coast" [syn: {run}]
     4: wield vigorously; "ply an axe"
     5: use diligently; "ply your wits!"
     [also: {plied}]

Source : Free On-Line Dictionary of Computing

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)