To make dirty, foul, or disgusting; to cover with filth.
From 'be-' prefix combined with 'filth' (dirt or foul matter). 'Filth' comes from Old English 'fylth', related to 'foul'. The 'be-' prefix transforms the noun into a verb meaning to cause something to become filthy.
The 'be-' prefix was perfectly suited to creating vivid, visceral verbs about transformation—'befilth' doesn't just mean a little dirty, it means becoming thoroughly degraded and disgusting, which is why you see similar verbs like 'besmirch' and 'besmear' in contexts demanding emotional intensity.
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