Surrounded by or in the middle of a group of people or things; in the company of.
From Old English 'onmang' combining 'on' (in) and 'mang' (mixture), with an archaic '-st' suffix added later. It's an older variant of 'among,' showing English speakers' preference for adding sounds to the ends of words.
The word 'amongst' is just 'among' with an old-fashioned '-st' sound tacked on—kind of like how some people say 'betwixt' instead of 'between'—and it's one of those relics showing that English speakers historically loved adding extra sounds to words for no real reason.
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