Source : Webster's Revised Unabridged Dictionary (1913)

Deathly \Death"ly\, a.
   Deadly; fatal; mortal; destructive.

Deathly \Death"ly\, adv.
   Deadly; as, deathly pale or sick.

Source : WordNet®

deathly
     adj 1: having the physical appearance of death; "a deathly pallor"
            [syn: {deathlike}]
     2: causing or capable of causing death; "a fatal accident"; "a
        deadly enemy"; "mortal combat"; "a mortal illness" [syn: {deadly},
         {mortal}]
     adv : in the manner of death; "he was deathly pale"