[back to the wall] or [back against the wall] {adv. phr.}
In a trap, with no way to escape; in bad trouble.
* /The soldiers had their backs to the wall./* /He was in debt and could not get any help; his back was against the wall./* /The team had their backs to the wall in the second half./
Compare: BETWEEN THE DEVIL AND THE DEEP BLUE SEA, LAST DITCH, ON THE SPOT, UP AGAINST IT.