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]