very surprising; difficult to believe synonym amazing She ran 100m in an astonishing 10.6 seconds. I find it absolutely astonishing that you didn't like it.Extra examples During this period London grew at an astonishing rate. It was an astonishing way for a team captain to behave. She coped with the press with astonishing skill for a 19-year-old. The painting was sold at auction for an astonishing $30 million. Then he had an astonishing piece of luck.