On the sly and Undercover

On the sly adverb - In a furtive manner.

Undercover and on the sly are semantically related. in secret topic. In some cases you can use "Undercover" instead the phrase "On the sly" as an adjective or an adverb, when it comes to topics like sneaky, underground, clandestine.

Undercover adjective - Undertaken or done so as to escape being observed or known by others.

On the sly and undercover are semantically related. Sometimes you can use "On the sly" instead the word "Undercover" as an adverb or an adjective.