'Uncleanness' in the Bible

So the Israelis who had returned from captivity ate the Passover with all who had consecrated themselves from the uncleanness of the nations of the land in order to seek the LORD God of Israel.

"The land you are entering to possess is a morally unclean land due to the moral uncleanness of the nations of the lands along with their abominations that has filled it from one end to the other with their impurities. So, therefore, do not give your daughters in marriage to their sons, nor marry their daughters to your sons, and under no circumstances are you to seek their well-being or their wealth, so that you may remain strong, enjoying the best things the land has to give, and so that you may establish an inheritance for your children forever.'