[Christ on a flying eagle, are you high?! should be Ciel's first thought because seriously, the stuff that's coming out of Sebastian's mouth? Isn't culture. Except he doesn't spend all that much time talking to people who're high, surprising considering his presence in the Underworld. Sebastian usually takes care of that part. Again, the irony of it all.
Against his wishes, Ciel does look a little alarmed when Sebastian starts in with the giggle-fit... until he figures out that Sebastian is having an actual giggle-fit, and then he just wants to punch him in the throat. Again.
It's amazing how many times he feels like resorting to violence in his day to day dealings with Sebastian.]
Excuse me. [Translation: Have you lost your mind?]