不得不

bùdébù
  1. have no choice or option but to
  2. cannot but
  3. have to
  4. can't help it
  5. can't avoid

CEDICT

Entry Composition

1 不得 bùdé
  1. must not
  2. may not
  3. not to be allowed
  4. cannot
2 不 bù
  1. no; not so
  2. (bound form) not; un-