It's against Google's policy. It's called keyword stuffing and will get you delisted from Google, which means your site wont even show up on Google anymore. A good rule of thumb is to only show googlebot what users of your site can perceive display.
If you show the information to people at some point, then you can argue you're not keyword stuffing I guess, but Google will delist you first and then you'll have to file an appeal.
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