Wordorigins.org

Partridge, E. and P. Beale (1984). A dictionary of slang and unconventional English: colloquialisms and catch phrases, fossilised jokes and puns, general nicknames, vulgarisms, and such Americanisms as have been naturalised. London, Routledge & Kegan Paul.

A superb source that focuses mainly on British slang, but which is also useful for Americanisms. (The 1984 8th edition is the one consulted for this site.)