1. a piece of underwear worn under a shirt or other clothing, typically sleeveless or with short sleeves

Examples

• He put on an undershirt before his dress shirt.
• The white undershirt was visible through his thin polo.
• She bought a pack of cotton undershirts for her son.
• His undershirt kept him warm under his work uniform.
• The undershirt absorbed sweat during the hot day.
• He always wears an undershirt under his button-down shirts.