Dictionary.app is a built-in app on Mac OS X. Unfortunately it does not have Arabic support.
The good news is that Arabic support can be added easily, just follow these steps:
Download Arabic-plugin app then Install it (This app just copy Arabic.dictionary plugin to /Library/Dictionaries directory).
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 thought right click, selecting "Lockup in Dictionary"or pressing on it with 3 fingers )