perniciousness 1. Grave harmfulness or deadliness. 2. The quality of being very injurious, mischievous or destructive. 3. A reference to being pernicious; destructiveness, ruinousness. Related 'death, dead; kill' units: -cide; lethal-; mort-; necro-; phono-; thanato-. Found on http://www.wordinfo.info/words/index/info/view_unit/1377/