If there are unspoken rules that can't be defined then they're not rules, they're made up bullshit.

They can be defined, but nobody does so explicitly. Consider the classic example of the order of adjectives.

You can have a big brown bag, but if you have a brown big bag, something sounds wrong. You can have an excellent blueberry muffin, but not a blueberry excellent muffin. You can meet your 27-year-old Ukrainian friend, but not your Ukrainian 27-year-old friend.

Unless you majored in linguistics or learned English as a second language, you probably never once even thought about the rules of adjective order in English (and in other languages the rules are often different!). But you know them, follow them, and people who don't follow them don't sound right to native English speakers.

And there are so many weird rules like this.

I knew you were gonna mention this one and I don't think it really qualifies as it's not really an unspoken rule, it's the pretty well known and strictly defined adjective ranking order.

English: Shibboleths _all_ the way down.