The story
- Old English · inherited fromfor sōþ“truly, for certain, forsooth”
- Middle English · inherited fromforsothe
- English todayforsooth
From Middle English forsothe, forsoþe, for sothe, from Old English for sōþ (“truly, for certain, forsooth”), equivalent to for (“for, by”) + sooth (“truth”).
Etymology from Wiktionary, CC BY-SA 4.0.