Does anyone knows how 万一 became a phrase with "just in case" meaning? Is it a shorthand of a longer phrase or chengyu?
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1It may help to consider that English has similar numerical/probability phrases: "in the thousand-to-one chance that ..."; "in the off-chance that ..." – Seralt Feb 11 '16 at 4:24
http://baike.baidu.com/subview/199191/8163691.htm gives a plausible answer: it refers to a remote possibility of a thing X happening. Sort of "among the ten thousand and one things that could happen, one is X." Far back in ancient and classical Chinese, the ten thousand things are a proverbial way to express all that happens in this world. But I did not find sources giving any specific classical source for 万一 meaning "just in case."
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2A further support for the answer would be the saying 不怕一萬,祇怕萬一 The ten thousand (regular scenarios) don't bother me; I only worry about the one in ten thousand (unlikely scenario). – monalisa Feb 4 '16 at 21:30