不得而知

bùdéérzhī
  1. unknown
  2. unable to find out

CEDICT

Entry Composition

1 不得 bùdé
  1. must not
  2. may not
  3. not to be allowed
  4. cannot
2 而 ér
  1. and
  2. as well as
  3. and so
  4. but (not)
  5. yet (not)
  6. (indicates causal relation)
  7. (indicates change of state)
  8. (indicates contrast)
3 知 zhī
  1. to know
  2. to be aware