The story
From a speech by British Prime Minister Neville Chamberlain in 1938, in which he stated that "peace for our time" would be the result of the Munich Agreement of 1938, intending to allay growing concerns that Nazi Germany posed a risk of starting another major European war. Invoked sarcastically, due to the outbreak of World War 2 only one year later.
Etymology from Wiktionary, CC BY-SA 4.0.