Merriam-Webster Collegiate Dictionary - ply

 I. verb  (plied; ~ing)  Etymology: Middle English plien, short for applien to ap~  Date: 14th century  transitive verb  1.  a. to use or wield diligently busily ~ing his pen  b. to practice or perform diligently ~ a trade  2. to keep furnishing or sup~ing something to plied us with liquor  3.  a. to make a practice of rowing or sailing over or on the boat plies the river  b. to go or travel regularly over, on, or through jets ~ing the skies  intransitive verb  1. to ap~ oneself steadily  2. to go or travel regularly  II. noun  (plural plies)  Etymology: 3~  Date: 1532  1.  a. one of several layers (as of cloth) usually sewn or laminated together  b. one of the strands in a yarn  c. one of the veneer sheets forming ~wood  d. a layer of a paper or cardboard  2. inclination, bias  III. transitive verb  (plied; ~ing)  Etymology: Middle English plien to fold, from Anglo-French plier, pleier, from Latin plicare; akin to Old High German flehtan to braid, Latin plectere, Greek plekein  Date: circa 1909 to twist together ~ two single yarns