"Nihau" is a Hawaiian word that means "good" or "well". It's often used in greetings like "Aloha kakahiaka, nihau?" which translates to "Good morning, how are you?"
It's possible you might be thinking of a different word or have heard it used in a context where it wasn't meant to be a direct translation of "hello". If you can provide more information about where you heard the word, I might be able to help you find the correct meaning.