Definition
Arduous describes the kind of difficulty that is felt in the body — a steep, long, exhausting effort, from the Latin arduus, 'steep, high'. An arduous climb, an arduous journey, arduous negotiations that drag on for months: the word stresses not cleverness but stamina, the sheer toll of keeping on. It belongs to the diligent and the stubborn, because what is arduous is survived only by those willing to persist long after the first effort would have stopped.
Examples
Collocations
an arduous journey·an arduous task·arduous training·long and arduous
Synonyms
grueling·laborious·strenuous·demanding·taxing
Antonyms
easy·effortless·undemanding
Word family
arduously (adverb)·arduousness (noun)
In TOEFL & IELTS
TOEFL and IELTS reading use 'arduous' for expeditions, training, and reforms — anything achieved only through sustained effort. In Writing and Speaking, 'a long and arduous process' is a polished upgrade over 'very hard'. Mind the stress: ARD-you-us, on the first syllable. It describes the effort, not the worker — a task is arduous, a person is diligent.