Deathly

wiktionary

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From Middle English dethlich, from Old English dēaþlīċ, equivalent to death +‎ -ly.

etymonline

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deathly (adj.)

Old English deaþlic "mortal, subject to death" (a sense now obsolete); see death + -ly (1). Meaning "deadly" (of poison, sin, etc.) is from c. 1200; that of "death-like, characteristic of death, resembling death" is from c. 1200, originally of complexion. Compare deadly. Related: Deadliness.