Documents from the past often mean words from the past. Noah Webster created his first dictionary in 1806, and such historical dictionaries are often filled with terms which have disappeared or whose usage has changed. Look for old editions or historical reprints of standard dictionaries for the meanings and usage of such words, or modern dictionaries which cover these historical terms.
Even within a single language, words and usage may vary by locality. Slang dictionaries and dictionaries with a regional approach are an excellent source for the meanings of words used in a specific region or in an uncommon way.
Samuel Johnson prepared one of the most influential dictionaries in the history of the English language. Originally published in 1755, this 1828 2nd edition combines the pivotal works of Johnson and John Walker. Digitally scanned and available online by Google Books.
Common English idioms from the 17th and 18th centuries may have a different meaning today. This alphabetical guide by E. M. Kirkpatrick will help you make sense of these peculiar and colorful expressions and phrases. A free version can also be viewed online in Google Books.
Published in 1985, this unique glossary by Richard Lederer Jr. contains words and phrases used during the Colonial period in America which have either dropped out of use or had their meanings change drastically over the years. A great resource for anyone reading early American documents.
Learn the lingo of the Middle Ages with this free online dictionary of strange, legal, feudal, chivalric, monastic, military and architectural terms of the medieval period.