gruelling

The adjective 'gruelling' originates from the now-obsolete verb 'to gruel', meaning to exhaust or punish. This verb likely draws its sense from the noun 'gruel', referring to a thin, weak food given as a punishment or to invalids. The connection is metaphorical: an experience that is 'gruelling' drains one's strength as thoroughly as a punitive diet of mere gruel would. The '-ing' suffix transforms the action of the verb into a descriptive adjective, solidifying its modern meaning of an activity that is severely demanding and exhausting.

The word is a simplex form derived from the verb 'gruel'. The verb 'gruel' (to exhaust or punish) is of uncertain origin but may be related to the noun 'gruel' (a thin porridge), from Old French 'gruel', of Germanic origin. The '-ing' suffix is an English adjectival suffix forming present participles and adjectives from verbs.