Good morning @
Dv8rs Samuel Clemens was working on and riding on a River boat and as by their nature, rivers move a lot of dirt, sand and whatnot by the currents flow creating sand bars but not all are visible.
With that being said, the front of the boats “River man” drops a measuring stick in th water to measure the waters depth to determine if they can traverse the water way ahead.
The “River man “ would call out the waters depth and a River bout needs two fathoms of water or 12 feet and 12 feet would be called out as “mark Twain “. A half Twain is 6 feet
Mister Clemens fancied that for his nom de plume.