Ass-butt
Castiel: Hey, ass-butt!
(Castiel throws a Molotov Cocktail of holy oil at Michael, who screams as he goes up in flames and disappears.)
Dean: "Ass-butt?"
– Castiel and Dean, 5.22 Swan Song
Made by Castiel, the term "ass-butt" has become widely used in fandom as a term of both abuse and affection. The word "Ass" is also known as another name for a donkey. Castiel could have called Michael and Lucifer a donkey's butt.
Episodes
5.22 Swan Song
As Dean is confronting Michael and Lucifer, to try and reach Sam, Castiel calls out "Hey, ass-butt!" at Michael (much to Dean's confusion) before throwing a molotov cocktail of holy oil at him, giving Dean his five minutes alone with Lucifer. Unfortunately Castiel is soon ripped apart on a subatomic level by Lucifer for his troubles, but he and his vessel are later reconstituted by God.
11.10 The Devil in the Details
'Ass-butt.' I-I-I still don't get that.
When Lucifer takes Sam to revisit some memories, he takes them to Stull Cemetery to the moment when Castiel molotov cocktailed Michael with holy fire. Lucifer admits to Sam that he still does not understand what "ass-butt" means, before Sam moves the conversation along. Later, when Dean and Castiel arrive to save Sam from the beating Lucifer is inflicting on him, Cass again uses the insult to get Lucifer's attention.
12.07 Rock Never Dies
As Lucifer is about to use his powers on Sam, Castiel swoops in with another "hey ass-butt" before hitting Lucifer with a guitar and knocking him into the drum kit.
