deadly
dead-ly
adjective
Extremely effective, intense, or severe; causing great harm or difficulty.
"He delivered a deadly accurate shot."
adjective
Causing or capable of causing death; fatal or extremely dangerous.
"The cobra's venom is deadly."
adverb
To an extreme or intense degree; extremely.
"She was deadly serious about it."
Thesaurus
Synonyms: virulent, toxic, venomous, devilishly, madly, deucedly, insanely, lethal, fatal, pestilent, pernicious, baneful