To do or try something in a sneakly way, to avoid attracting attention or suspicion, or being caught in the act of doing or trying it; without others knowing or being aware of it.
He surreptitiously left the room.
She surreptitiously ran away from her boyrfriend.
They surreptitiously smuggled the goods through.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi