Definition of "deadeningly"

Although it was, on occasion, disturbing to see powerful and complicated emotions and behaviors distilled into deadeningly dull diagnostic phrases […]

1997, Kay Redfield Jamison, An unquiet mind: a memoir of moods and madness

[…] as soul-deadeningly, eye-strainingly, sleep-inducingly boring as going through fourteen file boxes of sloppily photostatted bank records.

2008, Jonathan Nasaw, Fear Itself