Wordorigins.org

Room, A. and E. C. Brewer (2000). Brewer’s dictionary of phrase and fable. New York, HarperResource.

An excellent source for general trivia, including etymologies. Do not confuse this current edition with the original first edition, the text of which is in the public domain and available online for free. The first edition is over 100 years old and is not a reliable resource. (The 1995 15th edition is the one consulted for this site.)