Initially, it was quoted in context for any appropriate situation (ie "Oscar will not be joining us for lunch today" .."Good! Good! More for me and you")
However, over time this phrase was condensed to "Good! Good!" to express any form of delight or enjoyment.
Currently, the term is a simple "GOOD", most effective when yelled or used in inappropriate situations.
Please note, this is a very contagious expression that may begin in confusing drunken banter and result in thousands of people using it regularly. GOOD!
"What do you think of bailing on the lecture and heading to the uni bar..?"
"So I'll see you later, then?"