noun
General term for a place of confinement; often used in compounds like 牢獄 (prison) or 牢屋 (jail).
彼は牢に入れられた。
He was put in prison.
牢から脱走するのは難しい。
It is difficult to escape from jail.
taru-adjective
Obsolete taru-adjective meaning 'firm' or 'strong'; found in classical expressions like 牢として (firmly). Not used in modern Japanese.
See also: 牢として
古文では「牢として動かず」のように使われた。
In classical texts, it was used like 'firmly unmoving'.
The kanji 牢 originally depicts an enclosure for cattle, later extended to mean prison. The obsolete adjective sense 'firm' is derived from classical Chinese usage.