防不胜防

防不勝防
fángbùshèngfáng
(idiom) virtually impossible to prevent

CEDICT

the utmost care could not prevent it from happening; to be impossible to defend effectively

Wiktionary

Entry Composition

1 防 fáng
  1. to protect
  2. to defend
  3. to guard against
  4. to prevent
2 不胜 bùshèng
  1. cannot bear or stand
  2. be unequal to
  3. very
  4. extremely
3 防 fáng
  1. to protect
  2. to defend
  3. to guard against
  4. to prevent