Green’s Dictionary of Slang

a general term of opprobrium; the overriding implication is that of stupidity.

1915		Advertiser (Adelaide) 28 May 11/7: Mr Jewell (heatedly)— And you are a ‘mong’. I say what I mean. He is not a man. He is a thing (Uproar).
1980		New Societty 31 Jan. 225/3: Two popular words?in current use are ‘spazz’ (or spastic), and [...] mong’ (i.e., mongol).
1988		McGill Dict. of Kiwi Sl. 73/2: mong [...] a mild term of abuse.
2000		M. Manning Get Your Cock Out 77: Like a lot of Downys, the salacious mong was sex-mad. [Ibid.] 80: He banged off almost the second he was locked in the mongy’s wet velvet shitlocker.
2001		Annanova 14 Dec. 🌐 An 83-year-old Australian man stabbed his wife to death because she called him a mong. Albert Hilder Goodwin is believed to be Victoria state’s oldest killer.
2003		M. Haddon Curious Incident of the Dog 56: People used to call [...] the children at school spaz and crip and mong.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 466: ‘You fucking useless mongs!’ I shouted at the jury.
2011		Mail Online 20 Oct. 🌐 Fury as Ricky Gervais ridicules Down’s Syndrome children with ‘two mongs don’t make a right’.
2012		J. Fagan Panopticon (2013) 7: El fuckmong, I call him, old cunt-pus.
2021		C. Moran in Times Mag. 6 Nov. 9/1: Kids in the playground don’t really shout ‘Gaylord!’ any more. Or ‘Spaz’, or ‘Mong’, or ‘Lezza’, or ‘Flid’.