The Oxford English dictionary, www.oed.com.
http://www.wordorigins.org is a good site.
Tricky said:Here's a good one.
$30 to look up a few words is too much for my tastes.
...snip..
But it is very good - if there are just a few words you want researching drop me a PM and I'll use my subscription to the OED to get the info you require.