Definition of mucko-chummo in US English:

mucko-chummo

adjective

British
military slang
  • Excessively close-knit, familiar, or informal with one's colleagues.

Origin

1960s; earliest use found in Punch. From mucko + chummo.

Pronunciation

mucko-chummo

/ˈmʌkəʊˌtʃʌməʊ/