In a way that drenches; so as to drench or thoroughly soak something.
From 'drench' plus '-ing' (present participle) plus '-ly' (adverb marker). Stacks morphemes to convert a verb into an adverb describing manner.
Adverbs ending in '-ingly' are wonderfully productive in English—you can take almost any verb ending in '-ing' and add '-ly' to create an adverb, like 'drenchingly,' which sounds natural even if rarely used because readers understand the morphological pattern.
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