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).