The story
- Old English · derived frommidde
- Middle English · inherited frommiddes
- English todaymidst
From Middle English middes, midst, myddest (“middle”), from Old English midde, reshaped in Middle English phrases like in middes (“in the middle”) by analogy with adverbs in -(e)s; also compare Old English on middan, tōmiddes. Forms in -(e)st are probably due to influence of superlatives.
Etymology from Wiktionary, CC BY-SA 4.0.