Tajiks

A Turkic runic inscription in Southern Kazakhstan dating to 720 AD, refers to Arab invaders as "Taziks" (Tajiks)

The term Tazik or Tajik was originally applied to Arabs or their descendants in Ajam. The author of Hayat-i-Afghani says : "Of the many theories regarding the etymology of this word Tajik, the most probable is that which makes it a corrupt form of the Pahlvi word, Tazi, an Arab, a word still current in modern Persian with the same meaning. All the dictionaries give Tazi as meaning the descendants of Arabs in Persia or any other foreign country." 

Note that Ghorids were not Tajiks in the meaning of Farsiwan. They are called Tajiks because they claimed to be descendants of Zahhak, an evil Arab king of Persian mythology who had serpents growing out of his shoulders.