(When Crassus went to ashes, Carbo became dull. Crassus = fat, dull, carbo = ash)
Grammarian Sacerdos contributes this witticism for Terentius. It even might have been a popular saying in the learned circles. It refers into rivalry between two powerful orators of their time, C. Papirius Carbo Arvina (praetor at 85) and L. Licinius Crassus (consul at 95).
C. Carbo Arvina was a son of consul of 120 C. Carbo. His father during his career switched sides from Gracchian to optimate faction. He went even as far as being involved somehow (we lack details) of the murder of the younger Gracchus. This however, was not enough for the optimate side to trust him fully and it seems he was chosen to be a public scape goat for the murder of popular Gracchus.
From the optimate side the leading speaker was no other than L. Crassus, who was perhaps the most celebrated public speaker of his time. Cicero, who was a student of Crassus as youngster, described Crassus to be the best public speaker of all time in Rome until Cicero himself surpassed him. Even giving a benefit of a doubt, I think we can safely say that Crassus was one of the best speakers in Rome at his time. But also Carbo was a celebrated speaker and is counted also among the best ones ever in Rome.
The clash of the giants never saw outcome as Carbo the elder made suicide before the court process was over. His son, Carbo Arvina, took Crassus as his main enemy and was also a noted public speaker in his attacks against Crassus.
Crassus became censor at year 92, and died next year. This is the turning point the saying refers to. When Crassus died, Carbo Arvina lost his enemy and in years to come was left without burning desire to avange his fathers death. This took away the sharpest edge of his oratory and he became dull instead.