Copperhead: (to Superman) What are you going to do, boy scout? Short my sheets? Give me a wedgie?
Batman: Joker! I should have known you'd be in on this.
Joker: Really? I must be falling into a rut.
Batman: I could have escaped any time. But I thought I'd stick around and keep an eye on you clowns.
Joker: You're despiccable.
Lex Luthor: (To Ultra Humanite) Et tu, Humanite?
Et tu, Brute: these are the very words Julius Caesar said to Brutus after Brutus [literally] stabbed Caesar in the back. The words 'et' and 'tu' mean 'and' and 'you' respectively and bear the meaning of betrayal.
Joker: (To Batman) You're despicable.
This is the catchphrase of Sylvester the Cat, from the Looney Tunes cartoons, and are often uttered when his cunning plans have been foiled.