The story
- Old English · inherited fromswā þæt
- Middle English · inherited fromso that
- English todayso that
From Middle English so that, so þat, sa þat, swo þat, swa þat, from Old English swā þæt, equivalent to so + that. Cognate with Saterland Frisian sodät, West Frisian sadat, Dutch zodat, German sodaß, sodass.
Etymology from Wiktionary, CC BY-SA 4.0.