Origin and Meaning of the Slogan 'Keep Calm and Carry On'

Symbolism in the slogan 'keep calm and carry on'
You can find an array of gifts in the shops that begin with 'Keep Calm and ...' but do you know the real version of the slogan is actually 'Keep Calm and Carry On'?
Popular Takeoffs on 'Keep Calm'
With the popularity of the poster only scaling high, there are some popular parodies or imitations that have appeared on similar lines, viz.

"Keep Calm and Have a Cupcake" (with a cupcake icon)
"Keep Calm and Hate Apple" (with the Windows logo),
"Keep Spending and Carry On Shopping", etc.
Keep Calm and Carry On is a motivational poster dating right back to the Second World War. The poster was made to boost the confidence of the British who lived in fear of the possibility of an air attack by the Germans.

Although millions of copies were printed, hardly any were ever publicly displayed or even well-known till the year 2000. Many, at the time, felt these posters to be distasteful and waste. However, they now make great memorabilia from that era.
Origin and History
During World War II, people lived in constant fear of the inevitable. They feared an air attack or a gas attack by Hitler's troops. Around the mid of 1939, Britain's Ministry of Information felt the need to boost the morale and spirits of the public in case of a war disaster. Thus came into being the poster 'Keep Calm and Carry on'.
The first design of this poster was done in 1939 as a part of a publicity campaign and was generated along with similar other posters that read "Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory" and "Freedom Is in Peril. Defend It With All Your Might". It was the third of the three motivational posters printed at the time. Several others were made later on.
Each of these slogans were printed under the Tudor Crown on the poster stands for King George VI, symbolizing British monarchy. The printing of these posters was done and ready to be handed out on the outbreak of the war, which was predicted to begin once the USSR and Nazi Germans signed the Molotov-Ribbentrop Pact.
However, the first half of this period of war passed by without any bombing, so this poster remained in the storage. By the time the Germans began bombing, the Ministry realized that the public was not particularly fond of the posters.

The Ministry found themselves in a slushy situation, where it feared having to defend itself against accusations of stupidity and wastage at the time of war. Therefore, they had most of these posters pulped.
Design
Many wonder about the person who designed the poster and the font used on it. Sadly, both remain a mystery till date. Since there were no computers at the time, the posters were hand-designed and the civil servant, who designed this poster remains anonymous.
However, it is believed that the font comes closest to Avenir, while others believe it to be Gill Sans. Some have even made it into its own font called, what else, Keep Calm font, which can easily be purchased online.
Meaning
'Keep Calm and Carry On' was meant to be distributed post bombing or a major war calamity. Therefore, its main purpose was to keep everyone calm and distract their attention away from the war. 'Carry On' also encouraged them to keep them going through their usual routine as if there was no war going on.
Resurrection of the 'Keep Calm and ...'
In 2000, a couple found the poster at the bottom of a box of old books that they purchased at an auction, in an England bookshop. Not knowing its worth or origin, they had it framed and hung it in the shop. They soon began selling the prints of the poster. A little over a decade later, a few more place found more copies of the poster.
Due to its simplicity of text and design along with its flexibility to fit in for almost all situations from Monday blues to treacherous deadlines, it instantly appealed to the youth of the 21st century. This gave rise to a boom online with retail businesses selling personalized 'Keep Calm' merchandise, viz. cups, posters, t-shirts, etc. These companies came up with a wide range of slogans beginning with 'Keep Calm and ...' to fit every mood and fancy under the sun.