In a manner relating to or using the dative case; functioning grammatically as a dative construction.
From 'dative' plus the adverb suffix '-ly' (in the manner of), creating an adverbial form meaning in relation to dative grammar.
You'll rarely see this word outside of grammar textbooks because linguists almost always just say 'in the dative' rather than the awkward 'datively', but it exists for maximum grammatical precision.
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