gruelling

UK: /ˈgruːəlɪŋ/

Collins Concise English Dictionary © HarperCollins Publishers::

gruelling, grueling / ˈɡruːəlɪŋ/ adjective

1. severe or tiring

noun

1. a severe experience, esp punishment

Etymology: 19th Century: from now obsolete vb gruel to exhaust, punish

WordReference Random House Learner's Dictionary of American English © 2025

gru•el•ing /ˈgruəlɪŋ, ˈgrulɪŋ/   adj. 

1. very tiring:
   a grueling day at work.

Also,[esp. Brit.,] ˈgru•el•ling. 

WordReference Random House Unabridged Dictionary of American English © 2025

gru•el•ing  (gro̅o̅′ə ling, gro̅o̅′ling), adj. 

1. exhausting;
   very tiring;
   arduously severe:
   the grueling Boston marathon.

n. 

2. any trying or exhausting procedure or experience.

Also,[esp. Brit.,] gru′el•ling. 

• slang gruel punishment (noun, nominal), punish (verb, verbal) + -ing², -ing¹ 1850–55

gru′el•ing•ly, adv.