Q:

What does “Hold one’s back” mean?

There is the following sentence in the scene Director of FBI, H.A.L Tyson instructs his men in Jeffery Archer’s fiction, “Shall we tell the President”:

“So they (assassins group) think you’re dead, Andrews, and they have
  now played their last card. So we must hold ours back, you’re going
  to remain dead – for a little while longer” 
Mark (Andrews) and O’Malley (both FBI agents) looked at each other,
  both puzzled. - Page 296.

I lightly took “hold one’s back” for a run-of-the mill idiom, but I couldn’t find this phrase in any of Cambridge, Oxford, and Merrimu Webster Dictionary as an idiom, nor does Google NGram Viewer include.
What does “We must hold ours back” mean? Isn’t “Hold one’s back” a common idiom?

A:

The phrase in the book is:

So they think you're dead, Andrews, and they have now played their last card. So we must hold ours back. You're going to remain dead—for a little while longer.

They're using an analogy to a card game such as poker. They have put their last card down, or made their intentions clear.
But they're going to keep their last card a little longer, and keep their secret from the others: that Andrews is not really dead.

