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.
Those guys are all birds of a feather.
It came to light that the politicians and the contractors were in cahoots.
同類 is a neutral term for 'same kind' or 'similar people', while 一つ穴のむじな specifically implies a group with shared, often dubious, intentions.
ぐる means 'accomplice' and focuses on the conspiratorial relationship, whereas 一つ穴のむじな describes the whole group as being of the same unsavory kind.
Standard mixed kanji-kana spelling. The animal name むじな is commonly written in kana.
Kanji spelling for むじな (badger-like animal). Rare in modern usage; the kana form is preferred.
Alternate kanji for むじな. Very rare; the kana form is standard.
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.