a place where bad people gather or hide; a group of people who commit crimes
—After the local post office admitted that some of its workers were overcharging customers, the mayor called it a den of thieves and vowed to punish those in charge.

The temple in Jerusalem was surrounded by an outer courtyard, called the Court of the Gentiles. It had this name because non-Jews (Gentiles) were not allowed into the areas closer to the temple building. Jesus was angry to see that this courtyard was filled with merchants selling animals, to be used for sacrifices, and exchanging money, to be used for offerings. This meant that the only place where Gentiles could worship at the temple had become a noisy, crowded marketplace. Chasing the merchants away, he quoted from the Books of Jeremiah and Isaiah:

Jesus entered the temple area and began to drive out those who were selling and buying in the temple courts. He turned over the tables of the money changers and the chairs of those selling doves, and he would not permit anyone to carry merchandise through the temple courts. Then he began to teach them and said, “Is it not written: ‘My house will be called a house of prayer for all nations’? But you have turned it into a den of robbers [den of thieves]!” The chief priests and the experts in the law heard it and they considered how they could assassinate him, for they feared him, because the whole crowd was amazed by his teaching. (Mark 11:15-18)

Similar, related phrases using den are den of iniquity and den of sin, both meaning “a place of immorality.”