I am not sure why nobody have asked for this before but someone just asked me if Notes mail have a built in thesaurus function. After some searching around, I can only found two workaround for this.
1. Lotus Sandbox - Thesaurus (link)
This code requires that Microsoft word is installed on the same machine as the client since it accessed the Office Thesaurus functionality. It would not work without it and it might not work properly if someday Microsoft decided to change the way the implement thesaurus on Word.
2. Use Live Text to lookup to an online dictionary site.
This would be feasible for client's with direct Internet connection. CMIIW I don't think it will work if the client use an authenticated proxy to access Internet.
Perhaps someone have a better alternative than those two ?