In a depreciative manner; while tending to reduce value or express disapproval.
From depreciative (adjective) plus -ly adverb ending, following standard English patterns for turning adjectives into adverbs describing how something is done.
The doubling up of similar 'depreci-' words (depreciatively vs. depreciatingly) shows how English created multiple ways to express judgment and loss—our language is obsessed with degrees of disapproval.
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