When a male character is not wearing a shirt or other upper-body clothing, but is wearing pants, underwear or other lower-body clothing.
For female characters, use either the no shirt tag (if wearing a bra, swimsuit, or other upper-body underwear with lower-body clothing like pants or a skirt), or the topless tag (if not wearing anything at all on the upper body). If the outfit never required a shirt in the first place (such as underwear only), male characters should be tagged with bare chest or other appropriate tags instead.
Don't use nude, bare shoulders, bare back, or bare arms with this tag.