Back in the spring of 1939, it was an anonymous civil servant who was entrusted with finding the slogan for a propaganda poster intended to comfort and inspire the populace should, heaven forbid, the massed armies of Nazi Germany ever cross the Channel.
70 years later, this slogan has resurfaced and is now a common phrase seen and used everywhere . . . . due largely in part to people like victoria smith of sfgirlbybay (she is also mentioned here).