From Proto-Mongolic *turuxun (compare East Yugur turuun) with irregular metathesis not found in varieties outside of Mongolian proper and unexpected -ай (-aj).
туурай • (tuuraj) (Mongolian spelling ᠲᠤᠭᠤᠷᠠᠢ (tuɣurai) or ᠲᠤᠷᠤᠭᠤ (turuɣu)); (hidden-n declension)