From 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.] [1913 Webster]
As men may warm wax with handes plie. --Chaucer. [1913 Webster]
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. [1913 Webster]
And plies him with redoubled strokes --Dryden. [1913 Webster]
He plies the duke at morning and at night. --Shak. [1913 Webster]
3. To employ diligently; to use steadily. [1913 Webster]
Go ply thy needle; meddle not. --Shak. [1913 Webster]
4. To practice or perform with diligence; to work at. [1913 Webster]
Their bloody task, unwearied, still they ply. --Waller. [1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Plied \Plied\, imp. & p. p. of Ply. [1913 Webster]