At this; at this point or circumstance; in reaction to this (archaic).
Compound of here (Old English her) and at (from Old English æt). The form combines a locative or reference word with a preposition to create an archaic adverbial expression, common in Medieval and Early Modern English.
In old texts, 'hereat he laughed' or 'hereat the king was wroth' uses 'hereat' like a time-stop button—pointing to a specific narrative moment, which modern English would handle with clunkier phrases like 'at this point' or 'upon this.'
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