Definition of sly adjective from the Oxford Advanced American Dictionary

     

    sly

     adjective
    adjective
    NAmE//slaɪ//
     
     
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  1. 1(disapproving) acting or done in a secret or dishonest way, often intending to trick people synonym cunning a sly political move (humorous) You sly old devil! How long have you known?
  2. 2[usually before noun] suggesting that you know something secret that other people do not know synonym knowing a sly smile/grin/look/glance, etc.
  3. Idioms secretly; not wanting other people to discover what you are doing He has to visit them on the sly.
 
adverb He glanced at her slyly.
 
noun [uncountable]
See the Oxford Advanced Learner's Dictionary entry: sly