Dear Hal: Leo Marx defined "down in the bills" on page 29 of the 1967 Bobbs-Merrill edition of Huckleberry Finn (Indianapolis, New York) as "written down in the specifications (bills as in the phrase, fill the bill); thus, foreordained." The standard slang and regional English dictionaries do not list the expression. Your student could look at the Glossary and Reference List in the Mark Twain Library or Mark Twain Works editions of Huckleberry Finn (University of California Press, 2001 or 2003) for a list of standard resources. Like Marx, the glossary defines it as "Written down in the specifications, hence predestined, foreordained." Victor Fischer Mark Twain Project.