One who is adept in the exquisite art of Microsoft Excel. Frequently one with professional experience requiring extensive use of Excel (e.g., investment banking, financial analysis). Specifically the abilities to do things with Excel for which it was not intended (e.g., word processing) or create files quickly using arcane keyboard shortcuts are common traits of the Excel Ninja.
Chris: I want to fix this spreadsheet so that instead of having equations, the numbers are hard-coded.
David (with hesitation): Control + C, alt, e, s, v.
Chris: Truly your status as an Excel Ninja is beyond question.