What does Genesis 44:12 mean?

Joseph's estranged brothers (Genesis 37:24–28; 42:7–8) have agreed to be searched by Joseph's steward. Their intent is to prove that none of them have taken his master's silver cup (Genesis 44:7–9). They have foolishly agreed that if it is found on any of them, that man will become the Egyptian ruler's slave. They are unaware that the cup has been placed in Benjamin's sack, by the steward himself, on Joseph's orders (Genesis 44:1–6).

Now the sacks are searched in order of the men's ages, starting with the oldest. The steward is focused on looking for the silver cup which he himself placed. He makes a show of searching each sack, knowing what he will find when he gets to the last one. All of this would have been part of Joseph's instructions; all of this is part of Joseph's final test of his family.

As expected by no one—other than the steward—the "stolen" cup is found in Benjamin's possession. This was the youngest brother, and the one whom Jacob was terrified to risk by sending him into Egypt (Genesis 43:14). Thanks to the rash vow made earlier, he now looks to be lost. The brothers react with an expected combination of surprise and grief (Genesis 44:13).