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 ]
a. Like gruel; of the consistence of gruel. [ 1913 Webster ]