A Freudian Slip is when you say what you say something that you did not to intend to come out the way it did, typically caused by context or distraction

For example: consider you are hanging out with a busty friend of yours you say, I think you are the breast vs I think you are the best.

That is a Freudian Slip.

In the context of the top level post.

Father and Daddy are interchangable but "daddy" can be used sexually. Freud is well known for his fixation with sex which pins the joke down. Calling Freud - "Daddy" - has quite a few overlaps of humor in the slip.

Edit: Almost any edit is grammar or typo related for me.