In a way that makes something less lively, intense, or interesting; with a deadening effect.
From 'deadening' (adjective) + '-ly' (adverb-forming suffix). Follows standard English patterns for converting adjectives to adverbs expressing manner.
This is an adverb that describes the method or manner of boring someone to death—when something isn't just boring but does so in a particularly thorough, relentless way, like a deadeningly slow bureaucratic process.
Complete word intelligence in one call. Free tier — 50 lookups/day.