Brewter's Dictionary of Phrase and Fable
Although the hardcover version is updated and thus more extensive, it is also more expensive. The online version is based on the 1898 original but still of great use. Did you for instance know that:
- To whistle down the wind is to defame someone.
Although it is a bit silly, the Urban Dictionary is useful for looking up slang. And funny wordplay of course. What, for instance, is Deja Moo? The feeling that you've heard this bull before...
and while on the subject of words...
The Online Etymology Dictionary
Etymology means the study of the true sense of words. It comes from Greek etymon which means "true sense" and logos which, of course, means "word". The word entered English via Old French. This, I found with a quick search in the Online Etymology Dictionary.
Sources as given