Definition of Deadly

Adjective: deadly (deadlier,deadliest) ded-lee

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

Adverb: deadly ded-lee

As if dead
- lifelessly
(used to intensify) extremely
"deadly dull"; "deadly earnest";
- madly, insanely, deucedly, devilishly