hard outer shell
Refers specifically to the tough, hard outer shell of nuts, seeds, or similar items, as opposed to the inner skin or husk.
Peeling the hard outer shell of chestnuts is tough work.
クルミの鬼皮はとても固い。
The hard shell of a walnut is very tough.
Compound of 鬼 (oni, 'ogre, demon') and 皮 (kawa, 'skin, peel, husk'), suggesting a tough, formidable outer layer. The exact historical derivation is uncertain.