William L. Shirer, The Rise and Fall of the Third Reich
The Phoney War was a phrase coined by the American Press. Like so many vivid Americanisms it soon came to be adopted on both sides of the Atlantic. It has become firmly established as a name for the period of the war from the collapse of Poland in September, 1939, until the opening of Hitlers Western offensive in the following spring.
Those who coined the phrase meant to convey that the war was spurious because no great battles were being fought between the Franco-British and German forces. In reality, it was a period of ominous activity behind the curtain. In the midst of it all a strange accident befell a German staff officer. The incident gave Hitler a fright, and in the following weeks the German military plan was completely changed. The old one would have had nothing like the same chance of success as the new one attained.
But all this was unknown to the world. The people everywhere could only see that the battlefronts remained quiet, and concluded that Mars had fallen into a slumber.
B.H. Liddell Hart, History of the Second World War