The Oxford English Dictionary
What is the OED?
The Oxford English Dictionary aims to be the definitive dictionary of the English language. It not only contains definitions of every word in English, but also the history of each word, including (if possible) its first use.
Why is this dictionary so big?
You think this edition (the Compact version) is big? You should see the real thing! In it's un-condensed form, the OED is twenty volumes.