Definition of ply noun from the Oxford Advanced Learner's Dictionary

 

ply

 noun
noun
BrE BrE//plaɪ//
 
; NAmE NAmE//plaɪ//
 
 
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[uncountable] (especially in compounds) a measurement of wool, rope, wood, etc. that tells you how thick it is four-ply knitting yarn Word Originnoun late Middle English (in the sense ‘fold’): from French pli ‘fold’, from the verb plier, from Latin plicare ‘to fold’.
See the Oxford Advanced American Dictionary entry: ply