What's a little head?

A priest was walking down the street when a hooker came up to him and said: Hey, guy! How would you like a little head?
Confused, he asked: What's a little head?
She laughed and just kept on walking. When he got back to the church he asked one of the nuns: What's a little head?
 

To which she barked back: $20, just like in town.