Shibboleth
Etymology
From Hebrew shibbōleth, "ear of grain" (or "stream"). In the Book of Judges, the Gileadites used the word as a test: fleeing Ephraimites could not produce its "sh" sound and said "sibboleth," betraying themselves — and were killed at the river crossing. The grisly origin story is why the word now means any group-identifying test.
noun · /SHIB-uh-leth/
A word, custom, or belief that marks someone as belonging to — or outside — a group.
What it names
The invisible password of insiders — a bit of pronunciation, jargon, or opinion that quietly sorts "one of us" from "one of them."
Example
How you pronounced the neighborhood’s name was a local shibboleth — get it wrong and everyone knew you had just moved in.
Related idea
Password is a deliberately assigned secret; a shibboleth works passively, exposing outsiders through habits they cannot easily fake.