Definitions for undershirtˈʌn dərˌʃɜrt
Random House Webster's College Dictionary
an undergarment for the torso, typically of lightweight fabric with or without sleeves.
Origin of undershirt:
singlet, vest, undershirt(noun)
a collarless men's undergarment for the upper part of the body
Kernerman English Learner's Dictionary
a piece of underwear worn on the top of the body
Put an undershirt on to keep warm.
an upper undergarment with no collar, and with short or no sleeves, worn next to the skin under a shirt; a singlet or (British) a vest
a shirt worn next the skin, under another shirt; -- called also undervest