Expression of surprise or exasperation. Similar to "Goodness gracious." Never a popular saying, but was used sparsely in the 1950's and 1960's. Probably a forerunner of great googly moogly.
"Great Googa Mooga, can'tyou hear me talkin' to ya?"
The Temptations - 'Ball of Confusion'
When something is absolutely bonkers in size, so large in fact that words other than Googaloogahungasulagunundamagugalahuganslojaingugaunbelangustantuant cant describe it.
You: Holy god she has melons!
Friend: Yeah, its Googaloogahungasulagunundamagugalahuganslojaingugaunbelangustantuant!
You: What
Friend: Googaloogahungasulagunundamagugalahuganslojaingugaunbelangustantuant... haven't you heard that on Urban Dictionary?
You: No.
Friend: You haven't? Read it