un·der·shirt (?n?d?r-shûrt?)n. An upper undergarment, typically having short or no sleeves, worn next to the skin under a shirt.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.undershirt (??nd?????t) n (Clothing & Fashion) chiefly US and Canadian an undergarment worn under a blouse or shirt. Brit name: vest Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014un?der?shirt (??n d?r???rt) n. an undergarment for the torso, typically of lightweight fabric with or without sleeves. [1640?50] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.