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Old English domes dæg, from domes, genitive of dom (see doom (n.)) + dæg "day" (see day (n.)).
In medieval England it was expected when the world's age reached 6,000 years from creation, which was thought to have been in 5200 B.C. Bede, c.720, complained of being pestered by rustici asking him how many years till the sixth millennium ended. There is no evidence for a general panic in the year 1000 C.E. Doomsday machine "bomb powerful enough to wipe out human life on earth" is from 1960.