永不反悔的保证（无懈可击的不在现场的证据等）
a guarantee, alibi, etc. that can be trusted completely
Can you give me a cast-iron guarantee that the work will be completed on time? 你能向我确保按时完成那项工作吗？
Walker had a cast-iron alibi which meant he was nowhere near Coventry on the day he was murdered.
There are few cast-iron certainties in this situation.
There are no cast-iron guarantees when it comes to investing.
They are seeking cast-iron assurances that there will be no redundancies.
See also: cast-iron