noun which may take the genitive case particle 'no', noun
unopened; sealed
Describes an item that has not been opened and still has its original seal intact, such as a package, envelope, or product.
未開封の封筒が机の上に置いてあった。
An unopened envelope was on the desk.
この商品は未開封のまま返品してください。
Please return this product unopened.
Means 'uncivilized' or 'undeveloped', not related to opening or sealing.
Compound of 未 (mi, 'not yet') + 開封 (kaifū, 'opening a seal'). The reading is straightforward on'yomi.