The story
- Proto-Indo-European*seh₁-
- Proto-Italic*sēros
- Latinsērus
- Proto-Indo-European*swer-
- Proto-Italic*swerjos?
- Latinsērius
- Proto-Indo-European*h₃ed-
- Proto-Indo-European*h₃édosder.?
- Medieval Latinsēriōsus
- Old Frenchserieux
- Middle Englishseryows
- Proto-Indo-European*leyg-
- Proto-Germanic*līką
- Middle Englishseryowslech
- English todayseriously
From Middle English seriously, sereously, ceryously, seryowslech (“earnestly”), equivalent to serious + -ly.
Etymology from Wiktionary, CC BY-SA 4.0.