Entry 1 sense Webster, 1913 Undershirt /ŭn'-dẽrsh-ẽrtˌ/ · Un·der·shirt · IPA /ˈʌndərʃˌɜrt/ 01 n. A shirt worn next the skin, under another shirt; -- called also undervest. 1. A shirt worn next the skin, under another shirt; -- called also undervest. Also: undervest