PREJUDICE, n. A vagrant opinion without visible means of support.

Ambrose Bierce (1842–1914?) American writer and journalist

“Prejudice,” The Devil’s Dictionary (1911)
    (Source)

Originally published in the "Cynic's Word Book" column in the New York American (1906-05-30) and the "Cynic's Dictionary" column in the San Francisco Examiner (1906-06-20).