Short for
金城汤池 (jīnchéng tāngchí), "metal city wall and moat of boiling soup" — from 《
汉书·
蒯通传》, where it describes defenses so strong they cannot be attacked. Almost always seen today inside the chengyu
固若金汤 (gù ruò jīntāng), "as solid as metal-and-boiling-moat."
