(As) thick as thieves

Having a close, intimate friendship or alliance. Very close-knit; friendly; allied. Very close or friendly; sharing secrets. Very good friends.

Examples:

  • Anna and Beth are together all the time these days - they're as thick as thieves.
  • The guys who work in the warehouse are thick as thieves. They don't really socialize with anyone else in the company.
  • Mary, Tom, and Sally are as thick as thieves. They go everywhere together.
  • Jones and Cook had met at the age of ten and were as thick as thieves.
  • Grant went to school with Maloney, the other lawyer in town. They're thick as thieves.