also: うちわねた
noun
A joke or reference understood only by a small group, such as friends or colleagues. Often used in casual conversation.
あの話は内輪ネタだから、他の人にはわからないよ。
That story is an inside joke, so other people won't get it.
彼らの会話は内輪ネタばかりで、ついていけなかった。
Their conversation was full of inside jokes, and I couldn't follow it.
Compound of 内輪 (uchiwa, 'private, inside a group') and ネタ (neta, 'material, topic'). The word ネタ is a reversal of 種 (tane, 'seed, source').