Definitions of deadly:

• adjective:   involving loss of divine grace or spiritual death

  Example: "The seven deadly sins"

• adjective:   extremely poisonous or injurious; producing venom
• adjective:   (of a disease) having a rapid course and violent effect
• adjective:   causing or capable of causing death

  Example: "A deadly enemy"

• adjective:   of an instrument of certain death

  Example: "Deadly poisons"

• adjective:   exceedingly harmful
• adverb:   as if produced by death

  Example: "Deadly pale"

• adverb:   as if dead
• adverb:   (used as intensives) extremely

  Example: "Deadly dull"