ON THE ORIGINS OF THE ETHNONYM “TAJIK”
The origin of the term Tajik is associated with Arabs (called Tazi, Taji by the Persians). In the east there was great resistance to Islam, and any Persian who became a Muslim was contemptuously called "Tazik", "Tazig" or "Tajik" (meaning "like an Arab") by the rest of his fellows. Gradually Islam prevailed and the name has stuck becoming the major ethno-label.
Similarly the New Persian which Tajiks began speaking with the arrival of Samanid Muslim rule -- could not have been possible without Islam or Arabic influence, as it contains about 25% Arabic vocabulary and phrases. It's script is also Arabic. Both the term Tajik and the language they are now known by -- are post-Islamic phenomena.
Another interesting implication of the above is that Pashto is technically regarded as the largest Pamiri (Saka) language ...so if one accepts the modern classification that Pamiris are Tajiks (which is officially current in Tajikistan), then the inference is that Pashtuns too are a type of Tajik! Now that may seem horrifying and reprehensible to many ordinary and good folks and Tajik cheerleaders, but it is academically a sound argument and fact.
Furthermore even nowadays the Dardic people in the Pakistani regions of Chitral and Gilgit call all Pashtuns and Tajiks (i.e all eastern Iranians) as "Taji" -- this is an ancient vestige which still remains in current usage because these areas and their environs are cut off from mainstream development.
Finally, another fact which I consider useful in adding here: In the east, the Dehgan Persian aristocracy forcefully resisted Islam and clung to their old Zoroastrian creed with fervour for a good few centuries after the Arab invasion toppled Sassanian Persia. They even paid "Jaziya" to early Muslim rulers to continue their old way of life. The Dehgans, who were an old and established aristocracy, were immensely proud of their ancestral bloodlines and ancient Persian heritage. The modern Tajik converts included many Sarts (Persianised Turks, mostly merchants and craftsmen) whom the Dehgans looked down upon as upstarts and "bazaris". Therefore the Dehgans developed an aversion for the new fangled pejorative term "Tajik" as it represented both upstarts as well as converts. Although in the end Islam succeeded in prevailing overall, this animosity of "Dehgan vs. Tajik" still exists very strongly in the population of Tajik/Persian origin in Pakistani society. But many are at a loss to explain it, having lost sight of the reasons and motivations behind it. Dehgans in Pakistan, especially the Swatis, consider "Tajik" as an alien and insulting term yet they own the name "Dehgan" with pride. My work, as well as that of the late historian Professor Akhtar has both educated and surprised them.