On the sly - definition of on the sly by The Free Dictionary
sly (slī) adj. sli·er (slī′ər), sli·est (slī′ĕst) also sly·er or sly·est 1. Clever or cunning, especially in the practice of deceit. 2. Stealthy or surreptitious: took a sly look at the letter on the table. 3. Playfully mischievous: a sly laugh. Idiom: on...