empty threat
idle threat

A member of the audience asked, "What's the difference between an idle threat and a real risk?"

But he and I both knew also that idle threats work sometimes.

"For those who think this is an idle threat, guess again."

The company thought it was an idle threat until the youth said he would bring down part of the company's computer system - and did.

The others knew that Tom's words were no idle threat.

But she had a feeling it wasn't an idle threat.

From the dead tone of the other's voice, it was no idle threat.

From a member of the Hussein family, it was no idle threat.

Now the announcement of a price increase is often an idle threat.

It could be idle threats but children like my son are saying why would they want to do this?