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"to hear what one is not meant to hear," 1540s, from over- + hear. The notion is perhaps "to hear beyond the intended range of the voice." Old English oferhieran also meant "to not listen, to disregard, disobey" (cf. overlook for negative force of over; also Middle High German überhaeren, Middle Dutch overhoren in same sense). Related: Overheard; overhearing.