Ass-butt

Castiel uses a molotov cocktail of Holy Oil to momentarily banish Michael

Samifer: Castiel... Did you just molotov my brother with Holy Fire?

In 5.22 Swan Song, Castiel calls out "Hey Assbutt!" at Michael before throwing a a molotov cocktail of Holy Oil at him.

Castiel: Hey, assbutt!
Lucifer: 'Assbutt,' I-I-I still don't get that.

In 11.10 The Devil in the Details, Lucifer takes Sam to revisit some memories. At the memory of Stull Cememtry, Lucifer admits to being bemused by the term. Later when Dean adn Castiel arrive to save Sam from The Cage, Cas again uses the insult.

The word has become widely used in fandom as a term of both abuse and affection.