Jon gives the etymology here.
In summary, it means that the Fed is very close to acting.
When used at the Dec 2000 meeting, it presaged an inter-meeting move for example – but it has not done so every time.
I wonder why Jon posted this today?
This plus his earlier article on how central banks are “Holding Fire—for Now—but Laying Plans” suggests to me that the Fed is alarmed at the ECB’s fail last night, and willing to step in to support markets if confidence totally collapses and equities start tanking.