Very close friends who share secrets, etc.
OriginThis idiom originated in the 1800s’. At that time, thieves often worked together in gangs and were extremely close, telling each other everything and completely replying on each other. ‘Thick’ in this case means ‘very close’ or ‘closely packed’, for example thick hair, thick grass, etc. These were put together to make the phrase ‘thick as two thieves’ which was later shortened to ‘thick as thieves’.