How does Pip give away his real motivations for visiting Miss Havisham?

Pip slips and almost uses Estella's name.... Joe gets the picture.

“Yes, Joe; but what I wanted to say, was, that as we are rather slack just now, if you would give me a half-holiday to-morrow, I think I would go uptown and make a call on Miss Est—Havisham.”

“Which her name,” said Joe, gravely, “ain't Estavisham, Pip, unless she have been rechris'ened.”