Accountants use the word foot to mean adding a column of numbers. To crossfoot means to verify that the sum of the totals in various columns also agrees to a grand total.
For example, assume you have a table of numbers that shows the sales of five items for the past week. Column A consists of the names of the five items. Columns B through H show the sales of each item for the days Sunday (Column B) through Saturday (Column H). Column I reports each item's total sales for the week. After footing each of the eight columns B through I, a good accountant will verify that the total of the totals in Columns B through H agrees to the total of Column I. This last step is known as crossfooting.