From the same as 'Aran (in the sense of strength); the ash tree (from its toughness) -- ash.
see HEBREW 'Aran
H766. oren
I. אֹ֫רֶן noun [masculine] fir or cedar (Assyrian êrinu COTGloss, Mishna plural ארנים) Isaiah 44:14 ("" יַעַר עֲצֵי אַוֺן, תִּרְזָה, אֶרֶז,).