Please read the passage before reading the commentary
A casual friend does not give a casual friend underwear as a gift. God sent Jeremiah to buy a loin cloth, which is underwear, or perhaps a jockstrap or lingerie, something intimate. God must have been more than a casual friend with Jeremiah. Perhaps the underwear symbolized God’s people, who were skin-close to God who loved them well.
Then Jeremiah bought the underwear and following God’s instructions, hid it near the river. Some time later, God sent Jeremiah back to retrieve the underwear, but it had rotted and was worthless. Jeremiah could throw away the underwear after he had found it, but would God really put God’s people in the trash can?
God was closer to God’s people than underwear is to the one who wears it. God does threaten this to the people. God even complains that despite the anguished entreaty by God in the episode of Jeremiah but in the end, they did not listen.
My people, God wants to be closer to us than our underwear is to ourselves. God has bought us as God’s underwear and although we may think we have hidden ourselves from God and rotted away, God still wants us closer to God than our underwear is to us.
It is not easy to talk about God as underwear, but the symbolism is here. Underwear is clothing that always touches our skin, that always picks up our smell, and somehow identifies us. It is true that casual friends do not gift other casual friends with underwear, but God is not being casual with us because God desire deep intimacy with us; casualness is not on the table in God’s relationship with us. In fact, God calls us out of our casualness into God’s own deep intimacy.