1. a sudden, unexpected blow
be hit with a sucker punch
A 'sucker punch' is a violent blow or violent strike. The blow in question can refer to being physically struck by somebody unexpectedly, or a damaging defeat or setback. To strike somebody when they are not looking or expecting it is seen as a sneaky, cowardly, and dangerous act, and it is often done without any provocation. In this sense, the term is used exclusively in a social context. A setback or defeat that is a 'sucker punch' can be debilitating, even catastrophic due to it's unexpected and forceful nature, and can be applied to either a social or professional context. A similar term is 'to blindside somebody.' This is an informal expression.