You know you’re a total language geek when reading this paragraph induces a frisson of delight. In one paragraph, why it’s called both “octothorpe” and “pound sign”! I’m giddy.
Though it is now referred to by a number of different names—“hash mark,” “number sign,” and even “octothorpe,” a jokey appellation coined by engineers working on the Touch-Tone telephone keypad—the phrase “pound sign” can be traced to the symbol’s ancient origins. For just as “lb” came from libra, so the word “pound” is descended from pondo, making the # a descendent of the Roman term libra pondo in both name and appearance.
From The New Yorker (on the new book “Shady Characters: The Secret Life of Punctuation, Symbols & Other Typographical Marks,”