一言既出，駟馬難追

一言既出，驷马难追
yīyánjìchū,sìmǎnánzhuī
lit. once said, a team of horses cannot unsay it (idiom); a promise must be kept

CEDICT

Entry Composition

1 一言 yīyán
  1. one sentence
  2. brief remark
2 既 jì
  1. already
  2. since
  3. both... (and...)
=3 出 chū
  1. to go out; to come out
  2. to arise; to occur
  3. to produce; to yield
  4. to go beyond; to exceed
  5. (used after a verb to indicate an outward direction or a positive result)
  6. classifier for dramas, plays, operas etc
=3 出 chū variant of 出 chū (classifier for plays or chapters of classical novels)
4 駟馬難追 sìmǎnánzhuī words cannot be taken back once they are spoken