一ひとつ穴あなのむじな

Meanings

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.

あの連れん中ちゅうはみんな一ひとつ穴あなのむじなだ。
Those guys are all birds of a feather.
政せい治じ家かと業ぎょう者しゃが一ひとつ穴あなのむじなだったことが明あかるみに出でた。
It came to light that the politicians and the contractors were in cahoots.

Kanji

一one, one radical (no.1)
穴hole, aperture, slit
狢badger
貉badger

Related words

同どう類るい
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
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.

Written forms

一ひとつ穴あなのむじな
Standard mixed kanji-kana spelling. The animal name むじな is commonly written in kana.
一ひとつ穴あなの狢むじな
Kanji spelling for むじな (badger-like animal). Rare in modern usage; the kana form is preferred.
一ひとつ穴あなの貉むじな
Alternate kanji for むじな. Very rare; the kana form is standard.

Etymology

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.