Pronunciation: (är'jOO-us or, esp. Brit., är'dyOO-),[key] —adj. 1. requiring great exertion; laborious; difficult: an arduous undertaking. 2. requiring or using much energy and vigor; strenuous: making an arduous effort. 3. hard to climb; steep: an arduous path up the hill. 4. hard to endure; full of hardships; severe: an arduous winter.