The story
- Proto-Germanic · inherited from*sehstô
- Old English · inherited fromsiexta
- Middle English · inherited fromsixte
- English todaysixth
From earlier sixt, from Middle English sixte, from Old English siexta, from Proto-Germanic *sehstô. By surface analysis, six + -th (ordinal suffix).
Etymology from Wiktionary, CC BY-SA 4.0.