Webster's English Dictionary

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deadly

1. dead.ly \'ded-le-\ aj 1: likely to cause or capable of producing death 
   {a ~ disease} <(a ~ weapon} 2a: aiming to kill or destroy : IMPLACABLE {a ~ 
   enemy}  2b: highly effective {~ expose}  2c: UNERRING {a ~ marksman}  2d: 
   marked by determination or extreme seriousness  3a: tending to deprive of 
   force or vitality {a ~ habit}  3b: suggestive of death  4: very great : 
   EXTREME r capable of causing death. DEADLY applies to anything certain or 
   extremely likely to cause death; MORTAL usu. implies that death has already 
   occurred or will soon ensue {received a mortal wound} FATAL stresses the 
   inevitability of eventual death, destruction, or utter defeat; LETHAL 
   applies to something having the power of killing as a significant attribute 
   or often as the sole reason for existence SYN syn DEADLY, MORTAL, FATAL, 
   LETHAL mean causing o 
2. deadly av archaic  1: in a manner to cause death : MORTALLY  2: 
   suggesting death  3: EXTREMELY {~ dull}