arduus.

Disputed. * Perhaps from Proto-Italic *arðwos, from Proto-Indo-European *h₃r̥dʰwós, from *h₃erdʰ- + *-wós (whence Latin -uus). The development of the Latin term is slightly unusual, as the sequence *-rdʰ- typically produces Latin -rb. According to De Vaan, it is possible that *-w- in the pre-form may have prevented the development into a labial, thereby explaining the Latin term. * Alternatively, the linguist Nicholas Zair derives the term from *h₂r̥Hdʰ-ew-, the weak stem of a Proto-Indo-European u-stem adjective. According to this theory, the term would follow a development from *h₂r̥Hdʰ-ew- > *aradʰ-ew- > *arad-ew-, whence ultimately arduus. * The linguist Robert Woodhouse proposes that a sequence *Hh₃r̥dʰwo- could produce arduus via sound development of *HHC- to Latin aC-. Cognate with Latin arbor (“tree”), Welsh ardd (“high; hill”), Old Irish ard (“high; height”), Ancient Greek ὀρθός (orthós, “upright, straight”), Sanskrit ऊर्ध्व (ūrdhvá, “rising upwards, high”).