To be lecherous is to be full of strong sexual desire and to act on it, usually in an unpleasant way.
People who are lecherous are kind of obsessed with sex — they can't stop thinking about it or having it. This is a word that is used more often to describe men than women: lecherous men are called lechers. There is also a creepy flavor to this word. If you describe someone as lecherous, it usually isn't a compliment. A common use is talking about a lecherous old man who hits on younger women.