Via Starwars.com:

The Star Wars films aren’t lacking in the humor department. Several characters get one-liners and snappy remarks — it’s even implied for characters like Chewbacca and R2-D2 who can’t verbalize their emotions in words we understand. A fair amount of insults are sprinkled through the saga, too. For a droid programmed for etiquette and protocol, C-3PO in particular sure knows how to level taunts and rude remarks towards his friends. These are a handful of his best insults.

“Don’t call me a mindless philosopher, you overweight glob of grease!”
R2-D2 is on the receiving end of several of C-3PO’s biting comments. Artoo tolerates his golden friend and though we can’t understand R2-D2’s exact replies, it sounds like he gives at least as good as he gets. In this case, Threepio is yelling at Artoo about getting into the escape pod on the Tantive IV.

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