There is no single site. Apparently one was proposed, but it did not go anywhere and got deleted. There's nothing to stop you from proposing another etymology site on Area 51 (but there's no guarantee the site will actually happen either).
Some (but probably not most) etymology questions can be asked on Linguistics, as there is an etymology tag. I'm not sure what the policy is now (since everything relevant I found on their meta is old and not very highly voted), so your case may be worth bringing up there.
The best place to go will be the specific Language and Usage site, if one exists for that language. For example, questions about the etymology of English words can be asked on English Language and Usage (we also have an etymology tag). To minimize the chance of your question getting closed be sure to check Etymonline before asking.