It is a sound that means nothing at all. Or it is another word for nothing!
Person 1: What the hell are you doing!
Person 2: Hiff Hiff
Perdon 1 What the hell did you say?
Person 2: Hiff Hiff! It means nothing at all!
Etymology: Believed to be of Australian origin, describing a grunt of exertion, or perhaps a modification of "heave".
Transitive verb: "to hiff (something)":
1. To throw (something) in a cumbersome manner, often something heavy or uncooperative.
2. To dispose of something, often something that has exceeded its usefulness.
Rarely used intransitively, such as in "Dave hiffs".
"Peter hiffed the bale of hay at the cows."
"Sharon hiffed the broken toaster."