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English definition of “sly”

sly

adjective uk   /slaɪ/ (slyer, slyest) us  
deceiving people in a clever way in order to get what you want: He's a sly old devil - I wouldn't trust him with my money. [before noun] seeming to know secrets: "You'll find out eventually," said Mary with a sly smile.
slyly
adverb uk   /ˈslaɪ.li/ us  
She grinned slyly and refused to tell me where the money came from.
slyness
noun [U] uk   /ˈslaɪ.nəs/ us  

sly

noun uk   /slaɪ/ us  
on the sly If you do something on the sly, you do it secretly because you should not be doing it: He drives his mother's car on the sly while she's at work.
(Definition of sly from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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