You have seen the initialism NYKNYC. What does it mean?
NYKNYC means "not your keys, not your coins" or "not your keys, not your cryptocurrency." A private key is a non-tangible key that is not supposed to be given to other people. A public key is an address that may be given to other people.
You have to protect that private key because if anyone else gets their hands on it, they can use it to say they own your address, which means they own your coins.
The engraving of "am I not a woman and a sister?" was an effort by women to help show that slaves were human beings. The negative put emphasis on the humanity of the slave. For this quote "not your keys, not your coins," emphasizes a wallet (to avoid a website exchange that has your private keys).
The word "middleman" became a word in 1675; this was the same year the word "psychology" became a word. Globalization began in the mid 1500s. People thought about merchant ships and what other individual people were thinking and needed new words. Cryptocurrency exchange websites act as a middleman between the users (those who buy cryptocurrency) and the assets (e.g., Bitcoin).
Without your private key, you may not have access to your cryptocurrency. A website for cryptocurrency exchange acts as a middleman; it has private keys to have access to the coins of its users. A user of such a website does not have access to their private keys directly.
The phrase "not your keys, not your coins (or crypto)", highlights the exchange being a custodian of the cryptocurrency assets. This may discourage people from using an exchange/website or cryptocurrency altogether. A cold cryptocurrency wallet (e.g., tangible hardware) could enable a cryptocurrency owner to bypass the (negative aspects of the) concept of "not your keys, not your coins."
To read more about "not your keys, not your coins/cryptocurrency", see the following links:
To learn more about cryptocurrency, see these two articles:
How Do You Insure Your Bitcoin/Cryptocurrency Holdings?
If you want to buy a cryptocurrency wallet, check to see if it supports the ones you want before buying. You can browse them on Amazon here.
Protecting the recovery seed is essential for getting your cryptocurrency if your wallet is damaged. There are cryptocurrency wallet recovery plates made with titanium and steel capsules that can help protect your cold seed storage for your wallet.