From wikipedia
The
Know Nothing movement was a nativist American political movement of the 1840s and 1850s. It was empowered by popular fears that the country was being overwhelmed by German and Irish Catholic immigrants, who were often regarded as hostile to Anglo-Saxon Protestant values and controlled by the Pope in Rome.
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The origin of the "Know Nothing" term was in the semi-secret organization of the party. When a member was asked about its activities, he was supposed to reply, "I know nothing."
http://en.wikipedia.org/wiki/Know_Nothing
