Cousin Bill
(N) a person that is or creates an atmosphere of buzzkill. A person who always stays sober at parties and tells everyone not to drink. The origin of the phrase most likely comes from switching the first two letters of the syllables in buzzkill to kuzzbill, then lengthening it to said phrase. This phrase is mostly used in the presence of the Cousin in order to not hurt his/her feelings.
Sean: Cousin Bill sure is bringing me down tonight, she won't even have a Johnnie Bomb!
Nick: Tell me about it. The Cousin keeps telling me to stop drinking, what a drag!
Nick: Tell me about it. The Cousin keeps telling me to stop drinking, what a drag!