Question: what if a guy says he wants to eat you out?
Answer: Another Urban Dictionary result states, "Eating out is when a man, or in some cases a woman, performs oral sex on a woman, by placing his/her tongue/lips on the genitals (vagina, clit, pussy) and licking or nibbling on either." So does "eating out" come from the act of "licking" and "nibbling?" Potentially.