1What is contained inside something such as a box, bag, or container — the actual contents as opposed to the outer appearance.
引っ越しの荷物、中身は何が入っていますか。
What's inside these moving boxes?
このおにぎりの中身はツナマヨです。
The filling of this rice ball is tuna mayo.
財布を落としたが、幸い中身は無事だった。
I dropped my wallet, but luckily its contents were fine.
小包の中身を確認してからサインしてください。
Please check the contents of the parcel before signing.
2The substantive content of a talk, piece of writing, or plan — as opposed to its outward form.
会議は時間が長かったわりに中身が薄かった。
The meeting was long, but its content was thin.
あの候補者の演説は中身がない。
That candidate's speech has no substance.
タイトルだけでなく、中身で勝負する研究にしたい。
I want my research to stand on its substance, not just its title.
3(Specialized, somewhat dated) The blade portion of a sword, as housed inside its scabbard.
鞘から中身を抜いて手入れをする。
He drew the blade from its scabbard to clean it.