Toward the end of the first millennium A.D., Viking raiders struck terror in much of the western world, their long dragon boats appearing not only on the shores adjacent to the North Sea but as far afield as Sicily and the Sea of Marmara. Some established themselves on the Channel coast of what is today France, and in a later generation became the last conquerors of England under their leader, William.
The basic upper body covering for these men was the “sark,” a loose-sleeved shirt. But even this was too much for a plunder-bent Viking, and he typically went into action bare-chested, or “bar sark.” (It is from this phrase that we get our words “berserk” and “berserker,” the deed and the doer of frenzied activity.)
It cannot be stated with certainty, but is it too much to believe that it was the memory of these marauding, bare-chested sea raiders that gave rise to our modern-day “Keep your shirt on”?