Back the wrong horse — британская идиома, переводится как «Поставить не на ту лошадь.»

Вот как сами буржуи обозначают значение этой фразы:
1. To support a losing team — » My team is terrible, I backed the wrong horse.»
2. To support someone that can not win — «Tom backed the wrong horse at the meeting.»
3. «Hope we have backed the right horse».

Определение Кембриджского словаря:

Back the wrong horse — to make the wrong decision and support a person or action that is later unsuccessful.
Example: «In all his years as a book publisher, he rarely backed the wrong horse.»