once-in-a-lifetime
adjective [ before noun ] uk/ˌwʌn.sɪnəˈlaɪf.taɪm/ us/ˌwʌn.sɪnəˈlaɪf.taɪm/
一生中难得一次的；千载难逢的；难能可贵的
A once-in-a-lifetime experience or opportunity is very special because you will probably only have it once.
A tour of Australia is a once-in-a-lifetime experience. 澳大利亚之行是一生难得的一次经历。