In the corpora of republican authors and the glosses of late antique grammarians, the lexemes callidus and calliditas are used to describe a certain variety of intelligence, which is often translated into English as “cleverness” or “cunning.” This paper looks more closely at these lexemes in order to explain how the root call- (“hard”) came to be associated with mental capacity and acuity. In short, I argue that the type of intelligence that callidus originally denoted ought to be linked to the brutal treatment of slaves and the coping mechanisms that they had to develop in light of their condition as chattel. Not only is this violent form of education depicted in Plautus’ comedies, but its implications and logic can also be found in later authors such as Cicero.