[dead.ly] adj dead.li.er ; -est (bef. 12c) 1: likely to cause or capable of producing death
2. a: aiming to kill or destroy: implacable "a ~ enemy" b: highly effective "a ~ expose" c: unerring "a ~ marksman" d: marked by determination or extreme seriousness 3 a: tending to deprive of force or vitality "a ~ habit" b: suggestive of death esp. in dullness or lack of animation "~ bores" "a ~ conversation"
4: very great: extreme -- dead.li.ness n syn deadly, mortal, fatal, lethal mean causing or capable of causing death. deadly applies to an established or very likely cause of death "a deadly disease". mortal implies that death has occurred or is inevitable "a mortal wound". fatal stresses the inevitability of what has in fact resulted in death or destruction "fatal consequences". lethal applies to something that is bound to cause death or exists for the destruction of life "lethal gas".
[2]deadly adv (bef. 12c) 1 archaic: in a manner to cause death: mortally
2: suggesting death
3: extremely "~ serious"