Something deathly reminds you of death or of a dead body: a deathly pallor (a face with no colour), a deathly stillness, a deathly chill. It is about how a thing looks or feels — white, still, cold, emptied of life. As an adverb it does the same job: deathly pale, deathly quiet. People also stretch it toward “extremely,” especially with fear or silence: deathly afraid. Though actually, that stretch sits next to deadly, and the two are not twins. Deadly is about causing death (a deadly poison). Deathly is about seeming like death. A silence can be deathly; a poison is deadly.