Onomatopoeic. Describes the sound made by a Parakeet/Budgerigar when angry or threatened. Slightly higher pitched when threatened in various ways, be it from predators e.g. Cats or other birds, or indeed human contact with untame or young specimens.
The et-et noise sounds lower pitched and almost playful when the Budgerigar is angry, leading some fanciers to speculate that this does not indicate anger at all, and is more likely a sound of excitement.