Originally used within the kid's television cartoon series "Flintstones Kids" middle 10 minute featurette "Captain Caveman", this term is used as a descriptor for very startling observances and interactions: In most cases, gigantic, fat and ugly girls.
Most famously used in the phrase
UNGABUNGA CAPTAIN CAVEMAN!(obvious statement here)!
JEEZ look at the face on that... UNGA BUNGA dude!
Holy Ungabunga! she got run over by the stupid bus after falling out of the fat tree...