Present participle of begrime; the process of making or becoming grimy.
From begrime (verb) + -ing (present participle suffix). The -ing form describes ongoing action.
The '-ing' suffix is interesting because it's one of the few that can be both a verb form and a noun (gerund)—so 'begriming the factory' describes the action, while 'the begriming of the city' turns it into a concept.
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