"Sly" is already an adjective meaning cunning or secretive. Adding "ly" would make it an adverb, which describes how something is done, but "slyly" is already an existing adverb.
"Sly" is already an adjective meaning cunning or secretive. Adding "ly" would make it an adverb, which describes how something is done, but "slyly" is already an existing adverb.