Lucille is a “what”, from the comics. Not yet introduced in the TV Show. I won’t spoil, but if you search “Negan” and “Lucille” in your favorite image search engine you can see. Warning it may spoil some of the story also from the images.
Ah, I remember now. I think I saw a spoiler photo of the guy who is going to play Negan along with Lucille.......creepy