The letter Ḍād is the fifteenth letter in the Arabic alphabet. It’s one of the six letters added to the twenty-two pre-existing letters inherited from the Phoenician alphabet. It produces an extremely unique sound, and led the early Arabic grammarians to describe Arabic as the Lugat al-Ḍād, or the language of the Ḍād, since the sound was thought to be unique to Arabic.
Some took the letter Ḍād as a symbol of the Arabic language and, based on a (weak) ḥadīth narrated from the Prophet Muḥammad, the Arab people. The Prophet would have said that everyone who speaks the Arabic language is in fact an Arab, not by paternal or maternal lineage, but indeed because of the tongue. A recent example of a group combining this symbolism on a linguistic and ethnic level is the Arab Nationalist Guard, a secular pan-Arab nationalist militia fighting alongside the Syrian regime in the Syrian Civil War. Operating mostly around the capital Damascus and the southern parts of the country, their emblem is the Arabic letter Ḍād (ض).
Some examples featuring the letter Ḍād:
For more examples, please visit this thread on my Twitter.