exceedingly harmful
"The venomous snake delivered a deadly bite that threatened to stop the victim's heart within minutes."
(of a disease) having a rapid course and violent effect
"The outbreak was caused by a deadly virus that spread rapidly through the population with violent symptoms."
Subject to death; mortal.
"The ancient poison was deadly, causing even a single drop to prove fatal to anyone who ingested it."
as if dead
"The hunter stood deadly still, waiting for his prey to pass unnoticed."
(used as intensives) extremely
"she was madly in love"
"deadly dull"
"deadly earnest"
"deucedly clever"
"insanely jealous"
Fatally, mortally.
"The deadly virus claimed several lives within weeks of its discovery."
From Middle English dedly, dedlych, dedlich, from Old English dēadlīċ (adjective), from Proto-West Germanic dauþalīk, from Proto-Germanic dauþalīkaz ("deadly", literally "deathly"). By surface analysis, dead + -ly. Cognate with Saterland Frisian dodelk ("deadly"), West Frisian deadlik ("deadly"), Dutch dodelijk ("deadly"), German tödlich ("deadly"), Swedish dödlig ("deadly, fatal, mortal"), Icelandic dauðlegur ("mortal").