Definition of Pandora's box noun from the Oxford Advanced American Dictionary

 

Pandora's box

 noun
noun
NAmE//pænˌdɔrəz ˈbɑks//
 
[singular, uncountable]
 
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a process that, if started, will cause many problems that cannot be solved This court case could open a Pandora's box of similar claims. From the Greek story in which Pandora was created by the god Zeus and sent to the earth with a box containing many evils. When she opened the box, the evils came out and infected the earth.
See the Oxford Advanced Learner's Dictionary entry: Pandora's box