expression, noun
gang; group of accomplices; birds of a feather
Idiomatic expression literally meaning 'badgers of the same hole'. Used to refer to a group of people who are in cahoots or share the same (often shady) interests. Carries a negative connotation, implying they are up to no good together.
あの連中はみんな一つ穴のむじなだ。
Those guys are all birds of a feather.
政治家と業者が一つ穴のむじなだったことが明るみに出た。
It came to light that the politicians and the contractors were in cahoots.
From the idea that badgers (むじな) living in the same burrow are alike and likely up to the same mischief. The exact historical origin is uncertain, but it is a well-established idiom in Japanese.