萬無一失

万无一失
wànwúyīshī
surefire; absolutely safe (idiom)

CEDICT

Entry Composition

1 萬 wàn
  1. ten thousand
  2. a great number
2 無 wú
  1. not to have
  2. no
  3. none
  4. not
  5. to lack
  6. un-
  7. -less
3 一 yī
  1. one
  2. single
  3. a (article)
  4. as soon as
  5. entire; whole; all; throughout
  6. "one" radical in Chinese characters (Kangxi radical 1)
  7. also pr. yāo for greater clarity when spelling out numbers digit by digit
4 失 shī
  1. to lose
  2. to miss
  3. to fail