sneakily(in a sneaky manner) adverb [UK: sneak.i.lei] [US: sneak.i.lei]
stealthily(in a stealthy manner) adverb [UK: ˈstel.θɪ.li] [US: ˈstel.θɪ.li]While Mary gawked out the window, someone crept up stealthily behind her. = Mientras Mary miraba embobada por la ventana, alguien se le acercó sigilosamente por la espalda.