The term "ass-butt" has become widely used in fandom as a term of both abuse and affection. The word "ass" is another name for a donkey, so Castiel is calling Michael and Lucifer donkey butts. Alternatively, Castiel with his unfamiliarity with human vernacular is just putting two words he knows as insults together, resulting in a double bottom jibe.
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 ripped apart on a subatomic level by Lucifer for his troubles, but he and his vessel are later reconstituted by God.
'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 first used the insult and 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, Cas again uses the insult to get Lucifer's attention.