Someone or something out of date; has-been: Mr Stale is a back number(1880s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with back number
Dated, out of style. For example, That hat is really a back number, or The game has changed so fast that a player who returns to the circuit after several years' absence usually finds he or she is a back number. This term originally referred to back issues of periodicals, which are no longer newsworthy.
[ Late 1800s