Mark Twain was the pen name of Samuel Clemens. Clemens was a very clever writer. So it should come as no surprise the very name he chose for himself — Mark Twain — would be steeped in mystery and uncertainty.
Since time immemoriam sailors have marked the depth of by throwing a weight in the water at the end of a sounding rope with knots every 6 feet. The second knot was known as the twain, and it marked 12 feet, the minimum amount of water the paddle boats of Twain’s time needed without running aground. So when sailors yelled out ‘mark twain!’ for the captain, it was a moment of uncertainty. Was the boat moving into deeper, safer waters, or was it heading into shallower dangerous waters?