From WordNet (r) 2.0 (August 2003) [wn]:
gruelling
adj : characterized by toilsome effort to the point of exhaustion;
especially physical effort; "worked their arduous way
up the mining valley"; "a grueling campaign"; "hard
labor"; "heavy work"; "heavy going"; "spent many
laborious hours on the project"; "set a punishing pace"
[syn: arduous, backbreaking, grueling, hard,
heavy, laborious, punishing, toilsome]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
grueling \grueling\, gruelling \gruelling\adj.
1. characterized by toilsome effort; requiring extreme
physical or mental effort; as, a grueling campaign.
Syn: arduous, backbreaking, back-breaking, exhausting,
gruelling, hard, heavy, laborious, labourious,
punishing, slavish, strenuous, toilsome.
[WordNet 1.5]