from Old Anatolian Turkish Türk.
In Oghuz Turkic which is the parent branch of Turkish, Azerbaidjan and Turkmen languages, it reads as Türk with ü as in German. In Old Turkic, it was attested with two syllables as Türük which is the oldest attested form (cf. Orkhon inscriptions).
Although it sometimes refers to "Turkish, esp. muslim Turkish" in English and most of the European languages, it actually means
- any citizen of Turkish Republic according to the 1982 Constitution of Turkish Republic, Article 66, or
- a person from a community who speak Anatolian dialect of Turkish, or
- a person from a community who speak any dialect/branch of Turkic.
The reason that Europe (or the West) uses the former meaning related to the religion is because the Ottomans interacted with Europeans on behalf of the Muslim World. However, at the time of Ottoman, Turk meaned "coarse, countryman, villager" for the Ottoman dynasty eventhough they know that they have descendant from a Turkish father (cf. Kamus-i Turki
On the other hand, the major etnic group is rather called Oghuz but mostly Turcoman since Turk could refer etnically to different peoples like Kurdish, Armenians, Arabs, Greeks, etc. living in the Republican Turkey if they speak Turkish besides their own native dialect. Frequently Yörük, which literally means "walker, migrant", is used to refer Anatolian Turcoman etnicity because the Turcoman was known to native Anatolians as being nomadic or recently semi-nomadic.