Lecherously tells you how someone looks, smiles, speaks, or moves: in a lecherous way. The man is showing sexual wanting so that it feels unpleasant or annoying — staring, grinning, leaning in. It is the adverb, so it usually sits with a verb: look lecherously, grin lecherously, stare lecherously. The meaning is the same harsh one as lecherous; the -ly only marks that you are describing the action, not the person as a whole. It is not a common everyday word. You will hear it in writing, in film talk, and when someone wants to name that look exactly.