“一言既出驷马难追”的英语搞笑表达有：

One word go, jia jia jia jia.

You didadida me, I hualahuala you.

A word once spoken cannot be overtaken even by a team of four horses.

What is said cannot be unsaid.

A promise cannot be taken back once it is made.

A promise is a promise.

A word spoken is past recalling.

A word spoken is an arrow let fly.

这些表达都传达了“一言既出驷马难追”的意思，并且有些还带有幽默感，适合在轻松场合使用。希望这些表达能满足你的需求!