If you click on the Google Index tab on the side bar of GWT, you’ll see a link for Content Keywords.
The Content Keywords page lists the top keywords that are most significant to your site. This page shows you how Google interprets your website. You can click the “show more…” button to extend the list. If you find keywords on top of your list that is irrelevant to your site such as “Viagra”, it could mean that your site may have been hijacked by spammers or hacked. If this is the case, you can implement countermeasures for spammers.