Nonstandard or dialectal present participle of drown; a folk variation of drowning formed using different verb conjugation patterns.
From 'drownd' (dialectal past) plus -ing, or from direct analogy with other verbs; represents how non-standard dialects apply consistent rule patterns even when they diverge from standard English.
Speakers who say 'drownding' are actually being very logical—they're applying the same -ing suffix they use for all present participles, showing that dialect differences are rule-governed, not random!
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