Artha is a handy thesaurus based on WordNet with distinct features like global hot key look up, passive desktop notification, regular expression based search, etc. Artha may be used as a free open-source replacement/clone to the proprietary WordWeb Pro thesaurus (which is also based on WordNet) on UNIX-like and Windows operating systems.
Here is the link to its launchpad source : Source
How to Install
The package hasn’t been updated since marverik so you need to add Marverick source and update then install it.
Add these lines in your /etc/apt/sources.list
sudo gedit /etc/apt/sources.list
and copy paste it
deb http://ppa.launchpad.net/weboide/ppa/ubuntu maverick main
deb-src http://ppa.launchpad.net/weboide/ppa/ubuntu maverick main
once you have added the source the sources.list hit this comand from terminal
sudo apt-get update
sudo apt-get install artha
Done. Enjoy the offline dictionary!