囹圄 (língyǔ) is one of the oldest words for 'jail' in Chinese, found in 《
说文》 ('
囹,
狱也') and 《
华严音义》 ('
囹圄,
周之狱名也' — the prison-name of the Zhou dynasty).
身陷 means 'one's body falls into', so the whole phrase paints the literal image of a person sunk inside the ancient Zhou prison.