Lewdly is how someone speaks, looks, or moves when they are being crude about sex. A person who stares lewdly, or jokes lewdly, is putting that rude sexual tone on the act. It is the adverb of lewd, so it names the manner, not the whole person. Adults use it to disapprove. It is not soft playground slang, even if some groups laugh at that kind of talk. The meaning now is only this later “sex-rude” sense, not the old “unlearned” sense of lewd.