It hasn't been updated since it was bundled with NEXTSTEP and OPENSTEP.
Digital Webster from OPENSTEP
It was nice of Apple to put it back in, but the least they could have done was update the functionality.
For a Great thesaurus I suggest Nisus Thesaurus. When Apple didn't include the Digital Webster with Mac OS X, Nisus (and the Omni Group with OmniDictionary) not only brought back that functionality... they vastly improved on the concept.
Not that I'm complaining... at least Apple didn't steal the ideas the way they did with Watson.