I always thought that “mouth worked” describes when someone moves their mouth as if they are speaking, but no sound is emitted. This happens when they are so surprised that that they don’t know what to say.
He was shocked at his ex-girlfriend’s question. His mouth worked. He couldn’t answer.
Is this a common meaning of the phrase?
I did a google search and found this reference.
Also, what is the etymology? Did it evolve from another usage?