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Ever hear of an 'octothorp'? If you have a telephone, you see one every day. Octothorp is the technical term for the symbol #. It's called the "pound" sign on tedious automated phone-answering systems, but it's also known as a number sign, crosshatch, or hash mark. Legend has it that someone at Bell Labs came up with "octothorp" in the 1960s when touch-tone phones with asterisk and "pound" keys were introduced. It is more likely that it got its name from its use in cartography. A crosshatch used to indicate a village. "Thorpe" is Old Norse for "village" or "farm"; "octo" is Greek for "eight." Does # look like eight fields around a central square to you?

Source: 'Stumpers!' edited by Fred R. Shapiro (Random House, 1998); www.phrases.org.uk; 'Elements of Typographic Style' by Robert Bringhurst (Hartley & Marks, 2004).