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sly / slī/ • adj. (sly·er , sly·est ) having or showing a cunning and deceitful nature: she had a sly personality. ∎  (of a remark, glance, or facial expression) showing in an insinuating way that one has some secret knowledge that may be harmful or embarrassing: he gave a sly grin. ∎  (of an action) surreptitious: a sly sip of water.PHRASES: on the sly in a secretive fashion: she was drinking on the sly.DERIVATIVES: sly·ly (also sli·ly) adv.sly·ness n.