It literally means "dog's blood". It can be sometimes used like we use SOB in English, but it does not mean SOB. That would literally be "sukinsyn" in Polish. Your dictionary is not providing a literal translation, just a equivalent English phrase that sometimes, but not always, be used to convey approximately the same level of intensity and vulgarity are the Polish phrase. Sometimes other English phrases can or should be used instead, like GD, F or $hit or JC. It depends on the context.