Voltaire and Frederick the Great were friends and conversed in French, as all European aristocrats did at the time. And they were witty.
One day, Frederick invited Voltaire to come join him at his castle, Sanssouci, in Potsdam, by writing the following note:
_p__ à _ci__
Voltaire did not miss a beat and replied with his own note:
And they both began to look forward to their next meeting!
Voltaire immediately understood Frederick’s note to mean “venez souper à Sanssouci“–ie, come dine at Sanssouci. The word venez is “sous” (under) the letter p. The word sans is “sous” the word ci.
So he replied by saying J’ai grand appétit: Capital G = “Gé grand”; lower-case a = “a petit”.