Archaic or poetic term meaning 'at or in agreement' or 'in accord,' now essentially obsolete.
Middle English combination of 'a-' (preposition meaning 'at, in') + 'great' (past participle of agree, from Old French 'agreer'). This represents a frozen grammatical form from Middle English.
Medieval scribes wrote 'agreat' in business documents and contracts to mean 'agreed upon.' The 'a-' prefix is the same one you see in 'aboard,' 'alight,' and 'afloat'—English used to sprinkle it more liberally before standardization.
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