Against this; in opposition to this circumstance or statement (archaic).
Compound of here (Old English her) and against (from Old English ongeagn, meaning 'opposite' or 'in opposition'). The form represents a pattern where here could be combined with prepositions to create archaic adverbials expressing opposition or contrast.
Archaic English speakers could stack words like building blocks—'hereagainst,' 'therewithin,' 'whereunto'—allowing them to express complex relationships in single words, a technique that would make modern English sound archaic if we tried it today.
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