you never know with (someone or something)

you never know with (someone or something)

It's hard or impossible to predict someone's or something's behavior or quality. I think Jim is supposed to be finishing up that report, but you never know with him—he could have done a bang-up job on it, or he might not have done it at all. The TV is supposed to sync up with your phone through this USB adapter, but you never know with these cheap Chinese knockoffs.
See also: know, never
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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