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Adjective: gruelling  groo-u-ling
Usage: Brit, Cdn (US: grueling)
  1. Characterized by effort to the point of exhaustion; especially physical effort
    "a gruelling campaign"; "worked their gruelling way up the mining valley";
    - arduous, backbreaking, grueling [US], hard, heavy, laborious, operose, punishing, toilsome

See also: effortful