What a great story! A poster from 1939 created by a British civil worker charged with creating propaganda to comfort the most certainly stressed UK as they faced the Nazis. There were three and this particular phrase was the one seen because it was designed for the even of a Nazi invasion.
The poster was found again in the bottom of a box by a bookstore owner. He put it up and it is taking the world by storm. You can get shirts, hoodies, posters, screensavers and so on.
And, in the spirit of creativity – there are spoofs and variations. My favorite of these is:
Both slogans are great. I am going to try to take them to heart.