As firm as a rock.

　　eg：He will stick to his stand which is as firm as a rock.

　　中英俚语俗语来源：汉书："皆为金城汤池，不可攻也。 "

　　同义词：安如泰山 Safe upon the solid rock.

　　反义词：不堪一击 The earthen pot must keep clear of the brass vessel.