losing battle, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, a losing game. A failing effort or activity. For example, He's fighting a losing battle against putting on weight, or We think his candidacy is a losing game. These expressions, alluding to actual unsuccessful battles or games, have been used figuratively since the early 1600s. Also see lost cause.