Dictionary of Phrase and Fable
First published in 1870 Elizabeth Cobham Brewer's Dictionary of Phrase and Fable has become a classic source of information on allusions, idioms, mythology, place names, historical and fictional figures and events, nicknames, foreign terms, and famous phrases and quotations. Infoplease have just placed the 1894 edition online. There are approximately 16,000 entries and it is more than 100 years since it was written so "its tone and expression are frequently musty and antiquated, and its definitions and etymologies can be obscure and imprecise. But what this idiosyncratic reference work sometimes lacks in scholarly rigor is easily made up for by its wit, charm, and fascinating contents."
For a sample search - try this one on "mistletoe"
Thanks to Gary Price and Shirl Kennedy's Resource Shelf for drawing my attention to the availability of this online publication.