The word 'whacking' originates from the verb 'whack', an onomatopoeic term from the 18th century mimicking the sound of a solid strike or blow. By adding the participial '-ing' suffix, it formed an adjective describing something with the force or impact of a whack. Through figurative extension, this sense of forceful impact evolved in British informal usage to denote something impressively or exceptionally large. Consequently, as an adverb, it intensifies meaning to 'extremely', completing its journey from a sound-imitative verb to a common colloquial intensifier.

Structure: whack (to strike) + ing (adjective/adverb suffix) 'Whack' is of imitative origin, likely from the sound of a sharp blow. The suffix '-ing' is from Old English '-ende', '-inge', used to form present participles and adjectives/adverbs from verbs.