The adverb doubly means "twice as" or "double." If your new French class is doubly hard as last year's class, it's two times harder.
Sometimes people use doubly for general emphasis, to mean "very" or "especially." Your boss might say, "Today's doubly important because the owner of the company will be observing our work." Or you might feel doubly hungry for breakfast after a morning swim. Doubly comes from the adjective double, with its Latin root of duplus, "twofold or twice as much," from duo, "two."