[lewisflude]

To me, it means that the thing being designed is private in a way that can not be meddled with.

I.e. encrypted messages, that a company couldn't give to the government even if they wanted to.

In practical terms, this might look like building a system where you purposely don't store or give the ability to access personal content, instead relying on systems that are either local (i.e. not online), encrypted or otherwise fully obfuscated.