This is exactly what the Wikr bookmarklet does.
Follow the above link, and drag the Wikr bookmarklet provided there to your browser's toolbar. This creates a bookmark called Wikr which is actually the activation button for a useful javascript program. The next time you want to look something up while browsing any given web-page, highlight the term and then click on your Wikr bookmark. See also the "More info about" bookmarklet.