Lustful means full of strong sexual wanting, or showing that wanting in a look, a thought, or a way of acting. It is a heavy word. You meet it in older stories, in the Bible, and in writing that judges the feeling; it can sound moral, literary, or unkind. A younger speaker in ordinary talk is more likely to use simpler or rougher words. Though actually, lust once meant almost any strong wish, not only a sexual one, and in Old English lustful could mean 'full of desire' in a wider sense. Today's English has narrowed it. Built in a simple way: lust + -ful. It is not a polite compliment, and used about a real person it can land like an accusation.