The story
- Old English · inherited fromsōþfæstlīċe
- Middle English · inherited fromsothefastly
- English todaysoothfastly
From Middle English sothefastly, soþfasstlike, soðfæstlice, from Old English sōþfæstlīċe, equivalent to soothfast + -ly.
Etymology from Wiktionary, CC BY-SA 4.0.