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unstable (adj.)youdaoicibaDictYouDict[unstable 词源字典]
early 13c., "apt to move," from un- (1) "not" + stable (adj.). Similar formation in Middle High German unstabel. Meaning "liable to fall" is recorded from c. 1300; sense of "fickle" is attested from late 13c. An Old English word for this was feallendlic, which might have become *fally.[unstable etymology, unstable origin, 英语词源]