I have a brother in law. His name is Chuck. He's a very serious guy.
My neice (his as well) is very young and calls him uncle cuck. My wife's family is clueless and they also call him uncle cuck because they thought it was cute. My wife and I have to excuse ourselves when they are doing this, as we are about to die laughing.
Literally the entire family calls him uncle cuck and does not understand what it means, neither does he. It's amazing.