also grueling, "exhausting, punishing," 1852, present-participle adjective from gruel (v.) "to punish," from late 18c. slang get (or have) one's gruel "receive one's punishment," from gruel (n.).
Entries linking to gruelling
gruel n.
late 12c., "meal or flour made of beans, lentils, etc.," from Old French gruel "fine meal" (Modern French gruau), a diminutive form from Frankish *grut or another Germanic source, cognate with Middle Dutch grute "coarse meal, malt;" Middle High German gruz "grain," from PIE *ghreu- "to rub, grind" (see grit (n.)). Meaning "thin porridge or soup" is late 14c.