"Haisui-no-jin背水の陣" means fighting with your back to the water - a desperate last stand with no retreat possible. It's about giving your all when there's no way back. EX. 背水の陣（はいすいのじん）でこの戦い（てたかい）に挑んだ（いどんだ）。 I took on this battle with my back against the wall.