Back the field
Place a bet on all the horses in a race, except one.
This is the designation given to the betting against a particular horse (or small group of horses). If a punter believes a horse is sure to lose he can back all the others - 'the field'. This was referred to in the 1878 Chambers's Encyclopedia:
"There are men ... ready to lay against any horse and 'back' the 'field'."