Licentiously is the adverb of licentious. It tells you how someone behaves, lives, or writes: in a sexually immoral or uncontrolled way. The adjective paints the person or the life (a licentious king); this word paints the doing (he lived licentiously). It is formal and a little old-fashioned, like its base word. You will see it in novels and moral essays, not in ordinary chat. A dictionary line that defines it as if it were the adjective has mixed the two up: keep -ly for the way, not for the kind of person. It is simply licentious plus -ly, the usual English ending that turns an adjective into an adverb.