The phrase "staggering losses" refers to a significant and overwhelming amount of loss or defeat. It is used to describe a situation where the magnitude of the loss is so great that it causes shock, disbelief, or confusion. The word "staggering" implies that the loss has such an impact that it can make someone feel dazed, as if they are stumbling or swaying from side to side in surprise or dismay. In other words, "staggering losses" is a phrase used to describe a situation where the magnitude of defeat is so great that it takes your breath away and leaves you feeling overwhelmed.