shoptalk; insider talk
Originally theater jargon for talk that only makes sense to performers backstage; now used for any conversation or joke that outsiders cannot follow.
Their conversation was full of shoptalk, and outsiders couldn't understand it at all.
A joke that only a specific group understands, often because it relies on shared experiences or knowledge.
That joke was a complete inside joke, and no one else laughed.
Compound of 楽屋 (がくや, 'backstage dressing room') and 落ち (おち, 'punchline' or 'point'). Literally 'backstage punchline', referring to jokes or talk that only make sense to those familiar with the backstage context.