Against; in opposition to—an archaic or dialectal form of 'against.'
From Old English 'ongean' with the addition of '-st' (possibly a superlative or emphatic suffix), creating a variant of 'against' used in Middle English.
Words like 'ayenst' and 'ayen' show that 'against' is actually the simplified modern version of a much longer word—language is always simplifying itself.
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