zlacker

>>cosmoj+(OP)
Linguee is also great because it gives many examples of words in the context where they are used. (Context which isn't necessarily high quality though.)

And of course if you want to go deeper into a word, Wikipedia usually has its etymology... though I guess that for English that old Webster might be better ?

>>BlueTe+Cn
I've found Wiktionary to be far more objective than other dictionaries about its etymology. If there are multiple competing theories, it will say as much, instead of simply presenting one chosen possibility without qualifying it.