Slang meaning 'large,' 'many,' or 'very.'
Almost certainly from the Italian word Mondo, meaning 'world.'
It most likely comes to English by way of the 1961 shockumentary film Mondo Cane, meaning (roughly) A Dog's life (or more literally "world for a dog"). The film was rather extreme in tone, and the usage of 'mondo' to mean extreme (in various forms) may come from a misuderstanding of this title.
Mondo is often seen in the phrase Mondo Bizarro.
Thanks to sid for ier help and clarification.
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