inside joke; in-joke
A joke or story that only a close group of people (family, friends, colleagues) would understand, often referencing shared experiences. Colloquial.
Their conversation was full of inside jokes, and I couldn't understand it at all.
飲み会で身内ネタを連発するのは、新入りにはつらい。
Constantly cracking inside jokes at a drinking party is tough on newcomers.
Compound of 身内 (close relatives/insiders) and ネタ (colloquial for 'material', originally from 種). The exact origin is uncertain, but it likely emerged in casual speech to describe humor shared within a tight-knit group.