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. If you talk to some residents of Little Lamplight, they will sing the chant: 'Mungo, Mungo, smells like dung-o!'.

"Mungo" is likely slang for "Humongous," as grown-ups can often look quite large to children. It may also be a reference to Mungo Jerry but this is highly unlikely.