Deadly
Old English dēadlīc ‘mortal, in danger of death’ (see dead, -ly1).
wiktionary
From Middle English dedly, dedlych, dedlich, from Old English dēadlīċ(adjective); corresponding to dead + -ly. Cognate with Dutch dodelijk, German tödlich.
The adverb is from Middle English dedliche, from Old English dēadlīċe(adverb), from the adjective.
etymonline
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]
