Fo Guck Yourself

There's a guy who works here at our office who is somewhat up the managerial ladder from me, although he is not my boss. His name is Burt.

Every day when he comes in, I tell him: "Buck you, Furt".

This is an endless source of amusement for me, except that the other day he said "Juck you, Fim", and suddenly it became not funny anymore.