What does deadly mean?
"Deadly" describes something that causes or is capable of causing death — a venomous snake, a virulent disease, or a loaded firearm. From this literal core the word extends into figurative territory: a "deadly" rival is fiercely competitive, a "deadly" silence is oppressively tense, and a "deadly" dull speech is unbearably boring. It also serves as an intensifier, as in "deadly serious" or "deadly accurate." Derived from Old English roots relating to death, the word carries gravity and urgency in its literal use while remaining flexible enough for irony, humor, and praise — Australians, notably, may call something wonderful "deadly." Its most famous cultural appearance is in the "seven deadly sins," where the adjective marks vices considered spiritually fatal. Few words move so easily between menace and wit.
Causing or capable of causing death; fatal.
"The bite of the inland taipan is deadly without antivenom."
Also used figuratively for extreme intensity ("deadly serious") or extreme dullness ("deadly boring").
To an extreme degree; very accurately, effectively, or quietly.
"Her aim was deadly accurate."
Informal register; commonly paired with "accurate", "serious", or "quiet".
The phrase "deadly sin" dates back centuries — but in Australian slang, calling something "deadly" actually means it's excellent.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of deadly
"Deadly" derives from Old English "dēadlic," formed from "dēad" (dead) with the suffix "-lic" (having the form or nature of), meaning mortal or subject to death. It shares its root with the broader Germanic family of words around death, including Old Norse "dauthr." Over time its sense shifted from describing susceptibility to death toward describing the power to cause it, and it developed adverbial and figurative uses by the medieval and early modern periods.
Related word forms
How deadly is actually used
"Deadly" carries a strongly negative connotation in its literal sense but can be neutral or even positive figuratively — "deadly serious" implies intensity rather than harm, and in Australian English "deadly" is a compliment. As an adverb it appears mainly in informal speech ("deadly accurate", "deadly quiet"). The word also has legal weight: "deadly weapon" is a formal term in criminal law.
Easily confused with deadly
"Deathly" means resembling death (as in "deathly pale" or "deathly silence"), while "deadly" means able to cause death.