Past tense of 'congress'—when people or groups met together formally, especially in legislative sessions.
From congress (to meet together) + -ed (past tense marker). Though rarely used today, it's a valid archaic form showing congress functioned as a verb in earlier English.
Congress as a verb almost disappeared from English, but you still see it in historical documents—'they congressed' meant 'they held a congress,' showing how verb forms fade when their root noun becomes more common.
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