When Do You Join the Linguistic Community?

So one of the shorties was relaying with some agitation being told by a classmate that she was being rude when she was in fact being creative with no ill intent at all. She and a friend had developed a secret handshake of their own. They had seen others use the pinkie shake and decided that between them, the symbol of their friendship would be to extend their middle fingers towards each other and interlock them. This gesture, for them, expressed the proposition, "We are dear friends."

When a classmate witnessed the gesture, he told them that it meant something else, the propositional form of which he was not yet acquainted with, but of which he had sufficient knowledge to say that it was not the sort of sentiment one wanted to generally express in public. Shorty indignantly reported that said boy, as boys are wont to do, was just being difficult and interfering yet again with the girls just having fun on the playground.

So, we had a decision to make quickly over dinner. This is an aspect of the linguistic community to which Shorty has not yet been admitted. The gesture does not have the standard meaning for her that it has elsewhere. What do you tell her? TheWife's impulse was to say, "You can use any handshake you want." I went along, albeit a bit uneasily. After all, they will occasionally -- often when searching for rhymes with words like truck -- come across syllables we don't explain the meaning of, but inform them that it is "not a nice word." So, do you let it slide or bring her into the linguistic community here?