Realistically and figuratively: to gamble without having a chance of winning

You bet on the U.S.A. team to win the soccer world cup? Why put your money on a scratched horse?

Hai scommesso che gli Stati Uniti vinceranno il campionato mondiale di calcio? Perché puntare su un cavallo perdente?

Are you really going to vote for XY? Well, you'd better know you're backing the wrong horse

Davvero voti per XY? Beh, sappi che stai puntando su un cavallo perdente

A scratched horse is a horse that has been withdrawn form a race, so how can you win if you bet on a horse that is not running?