Does anyone knows how 万一 became a phrase with "just in case" meaning? Is it a shorthand of a longer phrase or chengyu?
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."