sly [slaɪ] ( compar slyer or slier, superl slyest or sliest ) adjective 1. [look, smile, grin] di chi la sa lunga 2. [cunning] furbo ( f furba) 3. [secretive] sornione ( f sorniona) sly [slaɪ] ( compar slyer or slier, superl slyest or sliest ) noun on the sly di nascosto