You just keep mum about it.

你就保持沉默。

Tom, we got to keep mum.

汤姆，我们得守口如瓶。

I reckon we're safe as long as we keep mum.

我想只要我们保守秘密，就不会有危险。

It don't make any difference what happens, we got to keep mum.

不管发生什么事，我们守口如瓶都一直不会变。

After another reflective silence, Tom said, "Hucky, you sure you can keep mum?"

汤姆又沉思了一会，然后说：“哈奇，你肯定不说出去吗？”