What did God tell Jeremiah not to let his new linen loincloth touch?

Answer: Water

He later hid it in the rocks at Perath (KJV: Euphrates). When dug up it was ruined, picturing Judah's ruined pride.

“This is what the LORD said to me: “Go and buy yourself a linen loincloth and put it around your waist, but do not let it touch water.””— Jeremiah 13:1 (BSB)