A secret political party of the United States, which arose in 1853, who replied to every question asked about their society, “I know nothing about it.” Their object was to accomplish the repeal of the naturalisation laws, and of the law which excluded all but natives from holding office. The party split on the slavery question and died out.
The chief principle of the party was that no one who had not been 21 years in the United States should be permitted to have any part in the government.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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