undershirt

From under- + shirt.

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undershirt (plural undershirts)

2. (fashion) An undergarment worn beneath a shirt, often collarless and sleeveless.
   • 2004 August, Menis Koumandareas, translated by Patricia Felisa Barbeito and Vangelis Calotychos, “Pale Face”, in Their Smell Makes Me Want to Cry (Birmingham Modern Greek Translations; 5), Birmingham, West Midlands: Centre for Byzantine, Ottoman & Modern Greek Studies, The University of Birmingham, →ISBN, page 19:

     And she’ll say, “Ooh, look at my baby’s undershirt! What sweet little sockies! What clothesies does my little one want?” ‘All perversion I see! And this lady, doesn’t she have any children?’

   • 2009, Peter Jungk, Crossing the Hudson, page 10:

     An undershirt absorbs moisture from your body and equalizes your body temperature.

As a rather generic term, undershirt is more often used in reference to male clothing, while female equivalents employ more specialized terminology.

• singlet, wifebeater (slang, men); vest (UK); see also underbodice (female equivalents)