Mungo

A Mungo is what the children in Little Lamplight call adults, since the time they were rejected by the inhabitants of Vault 87 telling them that they were "already dead" hence setting a grudge on all the residents of Vault 87 and all adults alike.

Origin
Possibly taken from the Mungo Man, an early human inhabitant of the continent of Australia, who is believed to have lived about 40,000 years ago. His remains were discovered at Lake Mungo, New South Wales in 1974. The remains are the oldest anatomically modern human remains found in Australia to date.

The name may also possibly be from the movie Blazing Saddles which features a character called Mongo. Mongo is portrayed as stupid yet strong, while clumsy and and large in size. The children in Little Lamplight accuse adults of being stupid, large and clumsy.