The term "Dumb Polack" was coined by the Germans in World War 2.
The Germans had surrounded the capital city of Warsaw with tanks and armament. They shelled the city to pieces and wanted the polish people to surrender. They refused and many fought to the death with pitch forks, shovels, anything in hand.
The Germans continued to shell the city unmercifully, so much so that the Germans called them "Dumb Polacks" for not surrendering even to overwhelming fire power and force.
A true example is one who would try to prevail against overwhelming odds. However, it is usually used as a derogatory term, without the user knowing its true meaning. Most people of character, would be proud to be called a dumb polack, if they knew its true meaning.