A good dictionary is the best place to look for information about the origins and meanings of words. The American Heritage Dictionary, for example, provides definitions and basic etymologies for more than 350,000 words.
The Oxford English Dictionary provides detailed and extensive etymologies for each word. The online edition is only available to subscribers. However, many public libraries have subscriptions that are available to any patron holding a library card.
The Historical Dictionary of American Slang is a useful reference for nonstandard words that may not appear in other dictionaries. It includes the earliest known usage of each slang word or phrase.