Where does it say rab in Harry Potter?

There is no mention of "rab" in the Harry Potter series. It's possible you are thinking of a different word or character.

Could you please clarify what you are referring to?

* Are you perhaps thinking of "R.A.B."? This is a mysterious initials mentioned in the books, referring to Regulus Arcturus Black, Sirius Black's younger brother.

* Maybe you are thinking of a specific character or scene? If so, please provide more details.

I can help you find the information you're looking for if you can tell me more about what you're remembering!