Mark Twain

From the Quicksilver Metaweb.

Mark Twain was the author of Huckleberry Finn's nom de plume. His real name was Samuel Clemens. His literary pseudonym comes from a signal given by a person measuring how deep the current is under a vessel on the everchanging waters of Mississipi River - "mark twain" (twain means 2 and is of the same root as between) was signal for deep water (2 fathoms, or 12 feet).