Dictionary app is a built-in app on Mac OS X. Unfortunately, it does not have Arabic language support.
The good news is that Arabic support can be added easily, just follow these steps:
1- Go to (System Preferences) on your computer.
2- Click on Security & Privacy > General > Allow apps downloaded from > anywhere.
3- Download the Arabic-plugin, Run it & follow the setup wizard.
4- Open the OS X Dictionary app.
5- From (Dictionary) menu select (Preferences) then enable Arabic option.
“You can even Drag (Arabic) to the top of languages to make it appear before other dictionaries. This way you will set (Arabic) as the default languages when you Lookup any word in the dictionary in your Mac (either through right click, selecting (Lockup in Dictionary)or clicking on it with 3 fingers)”