Admin Dijo:

دوست گرامی در دانشنامه اسلام چاپ لیدن (به عنوان یکی از معتبرترین منابع عمومی تاریخ اسلام)، جلد دهم، صفحه ی 62 زیر مدخل TADJIK اینطور آمده:
(جلد ده Desde aquí قابل دسترسی است.)

The traditional explanation of the term goes back
at least to E. Quatremere, Histoire des sultans mamelouks
de I’Egypte, ii/2, Paris 1845, 154-5, and was set forth,
p. ej.., in Barthold’s E71 art. This derives Tazik, etc./
Tadjik from the name of the Arab tribe of Tayyi1
[q.v.], Syriac Tayyaye, meaning “Arabs”, said to have
been the first Arab tribe encountered by the Persians
in pre-Islamic times (this would presumably be from
contacts with the Lakhmids [q.v.] of al-Hlra, who used
the Tayyi1 as frontier guards in ‘Irak, with lyas b.
Kablsa al-TaJI in A.D. 602 actually taking over the
wardenship of the marches from the Lakhmids), so
that the Persians then applied it to the Arabs in general.
The usage of the term may, however, be older
than the 6th century. It spread eastwards with the
Arab advance through Persia in the 7th century A.D.,
and when Arab troops reached Transoxania and first
encountered members of the Western Turkish empire,
the latter gradually took over the term, at first applying
it to all Muslims (between whose component
ethnic groups they did not as yet distinguish) but subsequently
to the Iranian peoples of Transoxania and
then Persia proper, as the Muslim people with whom
they were, by that time, most in contact.

توضیح سنتی برای اینواژه حداقل به سال 1845 بازمی گردد که لغت تاجیک از لغت عربی طایی آمده که نخستین قبیله عربی بودند که پارسیان در دوران پیش از اسلام با آنها تماس داشتندو بعدا پارسیان این را برای همه ی اعراب بکار بردند. در قرن هفتم میلادی با پیشروی اعراب در ایران آنها به ماورالنهر رسیدند و با امپراتوری ترک غربی مواجه شدند و آن زمان ببعد این لغت برای همه ی مسلمانان عرب یا غیر عرب و بدون توجه به قومیت آنها استعمال شد.

اما والتر هنینگ با این ایده موافق نیست:

W. Henning (cited in M. Mu’in’s ed. of the Burhdn-i
kdtic, Teherán, i, 455) therefore concluded that tdftik
in the sense of “Persa” has nothing to do with MP
tazik/g and MParth tdzik/g, which exclusively mean
“Árabe”, and convincingly postulated an origin for
tddj.ik in *Tdt-cik, originally *Tdd-cik. Persians migrating
from Fars to Transoxania would have brought
with them their own name for themselves and their
language, a name quite distinct from Tazik and Tazik,
names by which the Persians and Parthians respectively
called the Arabs.

هنینگ می گویید این لغت با لغت تازی که پارسیان به اعراب می گفتند تفاوت دارد و از لغت Tdd-cik آمده و گروهی از پارسیان بودند که از فارس به ماورانهر رفتند و نام خودشان را به همراه بردند.