I use such a thing every day on Firefox, an add-on called Dictionary Tooltip. I double-click a word; a small icon pops up near the word; I click the icon, and a small/resizable pop-up window appears that automatically searches any on-line website I have set it to. The pop-up is very fast, much faster than opening a new tab.
I have it set to Babylon Info by default, because that's very fast, and it auto-detects the language of the word. In a drop-down menu, you can have DT search other websites with a click. Several dictionary websites are built in, but you can add any custom website. You could add the OED if you have an on-line subscription to its website, or any searchable website, like StackExchange, if you want to quickly look up whether there exists a question about it. You can set DT to jump to certain headers on the page. Click anywhere outside the pop-up to close it. The pop-up has a button to open it (one time) as a tab in the browser, with the search results as a regular page.
This is the best search engine link to add the incredibly fast Babylon Info as a custom search website, which is rather difficult to find (it has many links of varying speed and quality). The
$$ in the link is what Dictionary Tooltip uses in place of the search string (it will only work as a bookmark in your browser if you replace $$ with %s). Babylon shows part of the Wikipedia article (with clickable link) if it can't find the word in any of its languages.
Both this and the following look-up were done with automated language recognition by Babylon, so no selecting necessary: just double-click the word, click the icon, and voilà. You can also set the pop-up to activate without having to click the icon, but this gives you more control. The settings menus are a bit awkward; try not to let that put you off.
As you can see, there's a small text ad at the top of the pop-up that apparently isn't blocked by Adblock; I am in way affiliated with this extension.
I don't think we really need this built into SE, unless it's either optional or completely unobtrusive and fast; the site is already very heavy on the CPU of this old PC I'm using, and I wouldn't want it to bog down even more. As it is, I am forced to type comments in an external editor, because they have terrible lag here.