expression, noun, noun which may take the genitive case particle 'no'
inside the walls; within prison
Literally 'inside the fence/wall', but often used as a euphemism for being in prison. Can also refer to any walled compound.
彼は塀の中にいる。
He is in prison.
塀の中の生活は厳しい。
Life inside the walls is harsh.
刑務所 is the formal term for 'prison', while 塀の中 is a more indirect, euphemistic expression.
Compound of 塀 (へい, 'fence, wall') + の (possessive particle) + 中 (なか, 'inside'). The prison meaning is a euphemistic extension.