I always though it was something very light; maybe the char.'s lucky boxers/undershirt/toga etc. But then again it might be their lighest favorite clothes.
When you roleplay/play what is under your character's armor? Is it clothing/underclothing/other. I thought of this while recalling other rogue-likes, in nethack you could wear a shirt under your armor. I also thought of the Angband ladder post on pants.