Source : Webster's Revised Unabridged Dictionary (1913)

Know-nothing \Know"-noth`ing\, n.
   A member of a secret political organization in the United
   States, the chief objects of which were the proscription of
   foreigners by the repeal of the naturalization laws, and the
   exclusive choice of native Americans for office.

   Note: The party originated in 1853, and existed for about
         three years. The members of it were called
         Know-nothings, because they replied ``I don't know,''
         to any questions asked them in reference to the party.