Green’s Dictionary of Slang

mungo n.2

[definition is queried in OED, as also is label ‘slang’]

? a person of position, a ‘swell’.

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Repository 8 June 75: But in the year 1770 the ladies of the first quality, the Mungoes, the superiors of the times, have abrogated [printed arrogated] the old Salic laws of libertinism, and openly set up a tavern in profest rivalry of Boodle’s, Arthur’s, and Almack’s .