A tank top typically worn under other clothes. Also known as a "wife-beater." You can't go out in just a boy-beater—put something else on.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. a sleeveless shirt or undershirt allegedly for women or homosexuals. (From the wife-beater.) They sat there looking macho in their tints and boy-beater.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.