A word once uttered cannot be recalled, even by a team of four horses. It emphasizes that once something is said, it cannot be taken back, and one must stand by their words.
既然你已经答应了帮助他，就应该全力以赴，一言出口，驷马难追。
Since you've already promised to help him, you should go all out, for a word once uttered cannot be recalled.
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