I like the idea that Gray and Cana, as kids who used to be as thick as thieves, developed coded messages for each other. And they both took it very seriously when it came to deciding whether or not they should let another person learn it too. Cana really wants to teach Juvia but Gray doesn't feel ready about it and keeps refusing. Juvia doesn't mind because honestly she's never seen Cana respect Gray's decisions before and she still can't believe it's happening.
But then Cana and Juvia end up doing their own language together.
-Cana claps her hands and shakes her butt- “Yes, Cana-chan. Juvia knows.” “The hell did she just do?” “Cana-chan wanted to remind me that she loves Juvia’s abs.” “What.”