This t-shirt is a play on the original “Keep Calm and Carry On” poster.
Keep Calm and Carry On was a poster produced by the British government in 1939 during the beginning of World War II, intended to raise the morale of the British public under the threat of impending invasion. It was little known and never used.
The poster was initially produced by the Ministry of Information in 1939 during the beginning of World War II. It was intended to be distributed in order to strengthen morale in the event of a wartime disaster. Two-and-a-half million copies were printed, although the poster was distributed only in limited numbers. The designer of the poster is not known.
(further reading: http://en.wikipedia.org/wiki/Keep_Calm_and_Carr...)