mongo, n. — Green’s Dictionary of Slang

mongo n.

[mong n.², but Mongo is a trad. name for a shambling idiot, often the servant of a ‘mad professor’ in films etc; however, that too may be rooted in SE mongol]

1. (US) an idiot.

1975		oral testimony in Lighter HDAS II.
1984		in ‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 247: The mongo’s died four times.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Mongo. Retarded prisoner.
2021		Breen & Conlon Hitmen 230: ‘I put it into first [gear]’ [...] ‘You did in your bollix put it into first, you fucking mongo, ye!’.

2. in attrib. use of sense 1.

2012

J. Fagan Panopticon (2013) 55: I cross my eyes, make a mongo face at him.

3. (US) scavenged valuables, e.g., saleable scrap metals.

1987

M. McAlary Buddy Boys 52: Sifting through their neighbors’ garbage for something they called ‘mongo,’ the garbage collectors came up with copper, other metals, and wood, which they later sold.