This article explains how and why the (monumental? ) tomb of Arghun Khan (r. 683– 690/1284– 1291), the fourth Ilkhanid monarch of Persia, was converted to the Islamic shrine of Qeidā r-e Payambar (Qedar the prophet), and gives a better justification for the origin and etymology of the name “ Qeidā r” , the supposed prophet/emā mzā da of the same shrine. The earliest occurrences of a “ Qeidā r-e Payambar” for that place are in Tā rikh-e Jahā ngir / Preface of afarnā ma (822/1419) of Š araf al-Din AliYazdi, and in afarnā ma (828-831 A. H. ) of the same author. From then on, Qeidā r has been generally identified with “ Qeidā r (Qedar) son of Ishmael” . This paper shows that “ Qeidā r” was, in fact, a toponym, and this toponym had its origin in the pre-classical Mongolian word for monastery, i. e. “ keyid” ; the monastery which, some years after burial of Arghun Khan in Mount Sujā s (modern Mount Qeidā r), was built on the site(? ) by his daughter, Uljatā yKhā tun. This research uses historical and geographical data, and takes a linguistic approach by etymological analysis of the toponym “ Qeidā r” .