萬無一失

surefire; absolutely safe (idiom)

surname Wan

ten thousand / a great number

not to have / no / none / not / to lack / un- / -less

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

to lose / to miss / to fail