The story
- Proto-Indo-European*dʰew-
- Proto-Germanic*dawjaną
- Proto-Germanic*daudaz
- Proto-West Germanic*daud
- Old Englishdēad
- Proto-Indo-European*leyg-
- Proto-Germanic*līką
- Old Englishdēadlīċ
- Middle Englishdedly
- English todaydeadly
From Middle English dedly, dedlych, dedlich, from Old English dēadlīċ (adjective), from Proto-West Germanic *dauþalīk, from Proto-Germanic *dauþalīkaz (“deadly”, literally “deathly”). By surface analysis, dead + -ly. Cognate with Saterland Frisian dodelk (“deadly”), West Frisian deadlik (“deadly”), Dutch dodelijk (“deadly”), German tödlich (“deadly”), Swedish dödlig (“deadly, fatal, mortal”), Icelandic dauðlegur (“mortal”). The adverb is from Middle English dedliche, from Old English dēadlīċe (adverb), from the adjective.
Etymology from Wiktionary, CC BY-SA 4.0.