Word Origins by 
The Facts on File Encyclopedia of Word and Phrase Origins by 
Good Word Guide
The Dictionary of Contemporary Slang
Chambers Dictionary of Eponyms
Rawson's Dictionary of Euphemisms and Other Doubletalk
Brewer's Dictionary of Modern Phrase and Fable
Merriam-Webster's Collegiate® Dictionary
Collins English Dictionary
The American Heritage Dictionary of the English Language
American Heritage Roget's Thesaurus
The American Heritage Dictionary of Idioms
The American Heritage Guide to Contemporary Usage and Style
The Penguin Rhyming Dictionary
The Browser's Dictionary of Foreign Words and Phrases
Collins French Dictionary and Grammar
Collins German Dictionary
Collins Latin Dictionary
Collins Spanish Dictionary