the.com/undershirt

means A light, usually sleeveless or short-sleeved shirt worn under an outer shirt, next to the skin, for warmth, modesty, or absorbing sweat.

from Straightforward compound carpentry: 'under' plus 'shirt,' built the same way as undercoat or underwear — English loves stacking a position word onto a garment to describe its layer in the outfit. 'Shirt' itself traces back to Old English 'scyrte,' related to 'skirt' (both from a Germanic root meaning a short garment), back when the word just meant a cut or short piece of cloth before it settled on 'thing worn on top.' The undershirt's job has stayed humble and unglamorous for centuries: sit closest to the body, catch what the outer layer shouldn't have to.