Etymology
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deadly (adj.)

Old English deadlic "mortal, subject to death," also "causing death;" see dead + -ly (1). Meaning "having the capacity to kill" is from late 14c. (Old English words for this included deaðbærlic, deaðberende). Related: Deadliness.

Deadly means that which inflicts death; deathly, that which resembles death. We properly of a deadly poison, and of deathly paleness. [Adams Sherman Hill, "The Principles of Rhetoric," 1886]

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Definitions of deadly
1
deadly (adj.)
involving loss of divine grace or spiritual death;
the seven deadly sins
Synonyms: mortal
deadly (adj.)
causing or capable of causing death;
a deadly enemy
Synonyms: deathly / mortal
deadly (adj.)
of an instrument of certain death;
deadly poisons
Synonyms: lethal
deadly (adj.)
extremely poisonous or injurious; producing venom;
Synonyms: venomous / virulent
deadly (adj.)
exceedingly harmful;
Synonyms: baneful / pernicious / pestilent
deadly (adj.)
(of a disease) having a rapid course and violent effect;
2
deadly (adv.)
as if dead;
Synonyms: lifelessly
deadly (adv.)
(used as intensives) extremely;
deadly dull
deadly earnest
Synonyms: madly / insanely / deucedly / devilishly
From wordnet.princeton.edu