Midst

google

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late Middle English: from in middes ‘in the middle’.

wiktionary

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From Middle English in middes(“in the middle”).

etymonline

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midst (n.)

"the middle; an interior or central part, point, or position," c. 1400, from Middle English middes (mid-14c.), from mid (adj.) + adverbial genitive -s. The unetymological -t is perhaps on model of superlatives (compare against).