Definition of "Deadly" :

adjective: (of a disease) having a rapid course and violent effect

adjective: causing or capable of causing death

"A deadly enemy."

adjective: of an instrument of certain death

"Deadly poisons."

adjective: exceedingly harmful

adjective: involving loss of divine grace or spiritual death

"The seven deadly sins."

adjective: extremely poisonous or injurious; producing venom

adverb: (used as intensives) extremely

"Deadly dull."

adverb: as if dead