secret handshake

plural secret handshakes

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. broadlyfigurativelyAny 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, Tom Goldstein, Jethro K. Lieberman, The Lawyer's Guide to Writing Well, page 19

   1 more example

   A signal that is not used by the population at large can be used by mutant members as a kind of secret handshake to identify an in-group. Mutants can then cooperate with those who share the secret handshake and defect against the others.

   2012, Jan van Eijck, Rineke Verbrugge, Games, Actions, and Social Software: Multidisciplinary Aspects, page 176