Not so much a greeting than a sentence opener. Had a classmate that always began a sentence with "Nin nah bei...." or "Kan ni nah...". I don't think it concerns the people around whatever it means, he just uses it out of habit and works as an attention grabber. Very vocal person.
When I learnt to talk I already know the greetings used by the male adults...." dieu nei ma chow hi!"...of course that young I don't know what it meant, thought it was some sort of greetings..