secret handshake

noun2

1. •
   A distinct form of handshake, often with the fingers or thumbs specially positioned, that covertly indicates a person's membership of a club or clique.
2. •
   (figurative) Any shibboleth or distinguishing sign.

   • Stuart Auerbach, who covered legal affairs at the Washington Post, has speculated that lawyers' language serves “as a secret handshake in a fraternity, letting others know you are one of the tribe.” — 2003