Suppose you suspect that you are going to be fired, but the boss has not officially told you yet - you spend the day in nervous anticipation, "waiting for the axe to fall" -- like a condemned prisoned who is waiting for the execution to proceed. I would just call it an expression - I'm not sure if there is an official name for it or not. Hope this helps.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.