Etymology of a Joke

You’ve probably heard about the time that Brutus, hungover after a long night of debauchery, had to sneak into the Senate, nearly XV minutes late.  As it happened, Cicero had the floor & was in full oration. 

“Psst (or the Latin equivalent),” Brutus, once settled in his seat, leaned forward, tapped Cassius on the shoulder & whispered.  “What’s the topic?”

“Dunno,” Cassius whispered back.  “He hasn’t come to the verb yet.”