"To back the underdog" means to support or root for someone or something that is in a weaker position or is expected to lose.
Sentences with «to back the underdog»
Casual bettors tend to overwhelmingly take the favorite, but they become increasingly willing to back the underdog in bowl games. (sportsinsights.com)
It's been a good season for publicly backed underdogs. (sportsinsights.com)
It's extremely rare to find the majority of public bettors backing the underdog, but that's exactly what's happening in this matchup. (sportsinsights.com)