Amongst

google

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Old English ongemang (from on ‘in’ + gemang ‘assemblage, mingling’). The -st of amongst represents -s (adverbial genitive) + -t probably by association with superlatives (as in against ).

wiktionary

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amongs +‎  -t( excrescent), from  among +‎  -s(genitive); surface analysis is  among +‎  -st( excrescent). Root  among from Old English ongemang, from  on(“in”) +  gemang(“assemblage, mingling”). 

etymonline

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amongst (prep., adv.)

"among," mid-13c., amonges, from among with adverbial genitive -s. The unetymological -t first attested 16c. (see amidst).