Offline Gnome Dictionary on Linux mint and Ubuntu:

The dictionary tool that comes with the gnome requires Internet connection to retrieve information from the dictionary server.So without an Internet connection it is useless.We can install a dictionary server on over local machine to avoid this constraint.
Installing Dictionary on localhost:
Copy and paste the following command to install dictionary server package.

#sudo apt-get install dictd

After installation of the above package you will need to install the database and English dictionary so enter this command.

#sudo apt-get install dict-moby-thesaurus dict-gcide

After the installation of above packages you can now use command line to consult dictionary as:

#dict -options ur_word

Configuring gnome dictionary client:
Now go to Office->Dictionary and then from 'edit' menu select 'Preferences'.Now click on 'add' button and enter following details in the 'add dictionary source' dialogue:

Dictionary name : local dictionary

Transport : leave default value

Hostname :127.0.0.1(loopback ip)

Port : (2628)leave as it is

After entering above details 'check' the radio button preceding 'local dictionary'.Now you can use your dictionary without an internet connection and with a faster response.
Regards!