Certain names can also be insults. Traditionally, 'Dick' is short for 'Richard.' It can also mean something totally different, but we don't need to explain that other meaning to you because you already know what it is.
We're really hoping that in the case of this receipt, either the customer or the server was called Dick. If they weren't, someone's been extremely rude. We're guessing it's a name thing, because if you were insulting someone, it's unlikely you'd pass on your thanks at the same time.