Guys, the word comes from "mongoloid" just like the British English term "mong". Chicos, proviene de "mongólico" o "mongolo". Used in US as well
"Deja de hacer el mónguer, tío."
The origin of the word monguer is not unknown, it comes from mongoose (in its derogatory meaning).
It is a word invented in social networks to be used instead of idiot, imbecile, delayed mental. Its origin is unknown.
Used in US as well