"arduous" in the adjective sense
1. arduous, backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome
characterized by effort to the point of exhaustion especially physical effort
"worked their arduous way up the mining valley"
"a grueling campaign"
"spent many laborious hours on the project"
"set a punishing pace"
2. arduous, straining, strenuous
taxing to the utmost testing powers of endurance
"his final, straining burst of speed"
"a strenuous task"
"your willingness after these six arduous days to remain here"- F.D.Roosevelt
difficult to accomplish demanding considerable mental effort and skill
"the arduous work of preparing a dictionary"
Source: WordNet® (An amazing lexical database of English)
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