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Old English ahte "owned, possessed," past tense of agan "to own, possess, owe" (see owe). As a past tense of owe, it shared in that word's evolution and meant at times in Middle English "possessed" and "under obligation to pay." It has been detached from owe since 17c., though he aught me ten pounds is recorded as active in East Anglian dialect from c.1825. As an auxiliary verb expressing duty or obligation (late 12c., the main modern use), it represents the past subjunctive.