Here "bare" does not necessarily mean naked, but to wear minimal covering such as undergarments. Alternate translation: "take off you fine clothes and make yourself unclothed" or "take off your fancy clothes" (See: [[rc://en/ta/man/jit/figs-explicit]])
The girding of loins with sackcloth is an act of grieving or mourning. "Gird your loins" is also a metonym, a command to prepare for work or battle, so this is an ironic way of emphasizing how miserable the people will be. Alternate translation: "put sackcloth around waists as you grieve" (See: [[rc://en/ta/man/jit/translate-symaction]] and [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/figs-irony]])