What does it mean?

Again, context is really necessary to give you a good answer. It seems like it maybe has some sort of entertainment context -- the script of a movie or TV show, perhaps. Or maybe it's about a horse that was injured, and was put down (killed humanely to spare its suffering), and someone thought that the horse would have been able to recover (that the killing was unnecessary).

Very hard to say without context (where you found that phrase, and what else was around it).

thank you
As a chinese girl ,I'm much obliged to receive your reply after I posted my question.If I did'nt express my thanks in time,please forgive me . Because I'm a college student,and we just use the computers at a fixed time in the class.
Now best wishes!