One of the mysteries of the English language finally explained.
1(A name given to) Ḥasan-e Ṣabbāḥ (d. 1124), founder of the Assassins, who established a base for the sect at Alamut, a mountain fortress near Qazvin in northern Iran, and his successors, especially Rāšid al-Dīn Sinān (d. 1192–3).
2A mountain guide or dweller. rare.
3A rock formation resembling the face of an old man. Chiefly as a proper name.
Late 16th century; earliest use found in John Frampton (fl. 1559–1581), merchant and translator. From old man + of + the + mountain, after Arabic šaiḵ-al-jibal.
Old Man of the Mountain/ˌəʊl(d) man ɒv ðə ˈmaʊntɪn/
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