The Collaborative International Dictionary of English v.0.48:

Deathly \Death"ly\, adv.
   Deadly; as, deathly pale or sick.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Deathly \Death"ly\, a.
   Deadly; fatal; mortal; destructive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "deathly":
   a la mort, abominably, agonizingly, appalling, awfully, baldly,
   balefully, baneful, bitterly, blatantly, blue, brashly, brutal,
   cadaverous, confoundedly, corpselike, cruelly, damnably, dead,
   deadened, deadly, death-bringing, deathful, deathlike,
   deathly pale, destructive, deucedly, distressingly, dolorously,
   dreadful, dreadfully, eerie, egregiously, excessively,
   excruciatingly, exorbitantly, extravagantly, fatal, feral,
   flagrantly, frightfully, ghastly, ghostlike, ghostly, grievously,
   grisly, gruesome, haggard, hellishly, horrible, horribly,
   improperly, inexcusably, infernally, inordinately, internecine,
   intolerably, killing, lamentably, lethal, livid, lurid, macabre,
   malign, malignant, miserably, mortal, mortuary, nakedly, openly,
   painfully, pale, pernicious, pestilent, pestilential, piteously,
   sadly, savage, shatteringly, shockingly, something awful,
   something fierce, sorely, staggeringly, terribly, to the death,
   torturously, unashamedly, unbearably, uncanny, unconscionably,
   unduly, unearthly, unpardonably, virulent, wan, wasted, weird,
   woefully