be secure against assault; be strongly fortified; impregnable/“他妈的，”徐鹏飞突然把开关一扭，关上了收音机：“什么～！连牛皮都不会吹！” (罗广斌、杨益言《红岩》596) “Damn them!” Xu Pengfei switched off the radio. “Strong as iron! They can’t even lie properly!”
固若金汤ɡù ruò jīn tānɡ
金：指金城；汤：指汤池。比喻所守的城池或阵地非常牢固。impregnable, strongly fortified, be secure against assault