same hole; living in the same den
Literally 'one hole'; used figuratively for people who share the same living space or are in the same boat, often with a negative or conspiratorial nuance.
They are birds of a feather (literally, badgers of the same hole).
Often used in the set phrase 一つ穴のむじな (badgers of the same hole) to mean a group of people with shared, often shady, interests.
Those guys are all in the same gang.
Compound of 一つ (one) and 穴 (hole). The figurative meaning comes from the idea of animals sharing the same burrow, extended to people who are in the same group or situation.