What does gilipollas mean?
Expression
prick — or, between friends, nearly nothing
One of the most common insults in Spanish, and genuinely offensive to anyone you do not know well. Between friends it runs affectionate or funny instead, which is the whole trick of this scene: the word does not change, the relationship does.
¿Pero tú eres gilipollas? ¡Cómo se te ocurre hacer eso!