What does Genesis 42:21 mean?

For a relatively brief three days, ten of Joseph's brothers are held in an Egyptian prison despite having committed no crime (Genesis 42:17). The brother they sold into slavery (Genesis 37:28) spent years in captivity and false imprisonment (Genesis 40:14–15), before ascending to become the second most powerful man in the nation (Genesis 41:44). When they come to Egypt during a famine, they fail to recognize him, and he holds them for a while before telling them they must return with their youngest brother. One of the ten will have to remain behind as collateral (Genesis 42:7–20).

Immediately, the men begin to discuss things among themselves. Joseph is still in the room. They assume he cannot understand them. He is using an Egyptian name and clothing (Genesis 41:42, 45) and to this point, Joseph has used an interpreter to translate between their Canaanite language and Egyptian (Genesis 42:23). This was part of his deception, of course. He grew up speaking their language.

That means Joseph fully understands when he hears his brothers telling each other their theory on why this is happening to them. Their immediate reaction is to assume they are being punished because of what they did to their "other brother" some twenty years earlier. Not knowing that very brother is standing before them, they express their guilt over seeing his distress and hearing him beg them not to sell him to the slave traders. They conclude their predicament is a result of the suffering they brought on Joseph all those years ago.

The first description of Joseph's sale into slavery in Genesis 37 did not note his reaction. Now we learn—unsurprisingly—that it was not a calm, stoic event. He begged them not to do it, and they would not listen. It's not surprising Joseph is struggling with how to respond to them now that he holds all the power over them. His emotions become difficult to contain as he listens (Genesis 42:24).