WordWeb Online

1. Causing or capable of causing death
   "a deadly enemy";
   - deathly, mortal
    
2. Of an instrument of certain death
   "deadly poisons";
   - lethal
    
3. Extremely poisonous or injurious; producing venom
   "deadly snakes";
   - venomous, virulent
    
4. (theology) involving loss of divine grace or spiritual death
   "the seven deadly sins";
   - mortal
    
5. Exceedingly harmful
   - baneful, pernicious, pestilent
    
6. (of a disease) having a rapid course and violent effect
    
7. [informal] So lacking in interest as to cause mental weariness
   - boring, deadening, dull, ho-hum [informal], irksome, slow, tedious, tiresome, wearisome, unamusing, draggy [informal], mind-numbing

1. As if dead
   - lifelessly
    
2. (used as intensives) excessively
   "deadly dull"; "deadly earnest";
   - madly, insanely, deucedly, devilishly

Derived forms: deadliest, deadlier

See also: fatal, irremissible, killing, lame, lameo [informal], meh [informal], noxious, samey [Brit, informal], toxic, uninspiring, uninteresting, unpardonable, virulent

Encyclopedia: Deadly, Unna?