Learn English – Word or phrase which indicates some shared knowledge

single-word-requests

I'm looking for a noun which is kind of word or phrase that is used to indicate the existence of some unspoken knowledge that the speaker has.

For example:

Her use of the the word "pincushion" was a deliberate ___________ intended to let him know that she knew his secret identity, without alerting those she suspected to be listening in.

I've a feeling this word begins with an S and has 3 or 4 syllables, but am drawing a long blank in my memory…

Best Answer

Shibboleth comes close, though it's not used in the way you intend. Historically, it was used as a proxy test related to whether the person started the word with the sh- sound or the s- sound.

Shibboleth noun A custom, principle, or belief distinguishing a particular class or group of people, especially a long-standing one regarded as outmoded or no longer important: liberal shibboleths about education - ODO

A shibboleth ... in its original signification and in a meaning it still bears today, is a word or custom whose variations in pronunciation or style can be used to differentiate members of ingroups from those of outgroups. Within the mindset of the ingroup, a connotation or value judgment of correct/incorrect or superior/inferior can be ascribed to the two variants. - wikipedia

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