There are two ways to do this that I know off:
Install Artha, an off-line thesaurus based on WordNet
sudo apt-get install artha
Or, you can configure Dictionary to use a local dictionary server
Install a local dictionary server and required dictionaries and thesaurus:
sudo apt-get install dictd dictzip dict-wn dict-foldoc dict-jargon dict-gcide dict-moby-thesaurus
Configure Dictionary to use the local dictionary:
Application > Office > DictionaryAdd a new source using
Edit > Preferences
127.0.0.1
as the Hostname
and select it as your default source.Hope that helps.