|informal, derogatory an ignorant person|
A party opposed to the holding of public office by immigrants or Roman Catholics. The Know-Nothings, also known as “nativists,” insisted that only true, “native” Americans should serve in the government. The party was quite successful in the 1850s but split over the slavery question. Its official name was the American party. It picked up the “Know-Nothing” tag because its members, maintaining secrecy about the party's activities, customarily answered questions with, “I know nothing.”
Note: Today, the term know-nothing is usually applied to bigots.