We find God using an object lesson to describe how the people of Israel have treated him. The loincloth would basically be our underwear and it would in one sense describe an intimacy with God that was unmatched. Israel was described as clinging to God. You see that example in vs.11 where God was hoping the people of Israel would be for God "a name, a praise, and a glory, but they would not listen." But because the loincloth was placed in an environment where it was ruined, so the people of Israel were ruined because they chose to run with the wrong nations. They chose to run with the wrong gods. They chose to turn their back on the God of Israel, who had chosen them, and they chose the gods that were in their midst from the other nations.
We see the author letting Israel know that how they are now they are not going to be able to change. Look at vs.22 and following where God says that they are not able to do good because they are so accustomed to doing evil. The end of this chapter gives us this phrase: "Woe to you, O Jerusalem! How long will it be before you are made clean?"