Definition of as common as muck in US English:

as common as muck

phrase

British
informal
  • Of low social status.

    • ‘You know, Ramirez, sometimes you seem as common as muck, and other times you're the most princely person I've met.’
    • ‘She is posing as a lady but she is really as common as muck.’
    • ‘He's as common as muck, and God help him if he has to perform state duties - he can't stand foreigners.’
    • ‘Sean Connery comes from the East of Scotland, with a 'posh' kinda tone, but we are from the West side and sound as common as muck.’
    rabble, scum, refuse, garbage, rubbish, trash, vermin, the lowest of the low, in the underclass, the dregs of society, good-for-nothings, undesirables
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