万一
1
por si acaso;en caso de emergencia
1
(unlikely event of) emergency;the worst(-case scenario);10000 to 1
万一の場合に備えて貯金するのは賢明だ。
It is wise to save money for a rainy day.
2
(if) by some chance;by some possibility;in the unlikely event that
万一彼がこちらへきたらすぐ知らせます。
If by any chance he comes here, I'll let you know at once.