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Online Etymology Dictionary

"The basic sources of this work are Weekley's 'An Etymological Dictionary of Modern English,' Klein's 'A Comprehensive Etymological Dictionary of the English Language,' 'Oxford English Dictionary' (second edition), 'Barnhart Dictionary of Etymology,' Kluge's 'Etymologisches Worterbuch der Englischen Sprache,' Ayto's '20th Century Words,' and Chapman's 1Dictionary of American Slang.' "
Oxford Etymological Dictionary

Searchable dictionary of English-language word origins.
Roots of English Dictionary

A free, downloadable software package that "will help you find the Greek and Latin roots of thousands of words in the English language. . .the hypertext links make it fast and easy to use. The Dictionary also contains background on the history of English and details on the sources of individual words."