Keith Houston’s fascinating essay The Ancient Roots of Punctuation begins like this:

The story of the hashtag begins sometime around the fourteenth century, with the introduction of the Latin abbreviation “lb,” for the Roman term libra pondo, or “pound weight.” Like many standard abbreviations of that period, “lb” was written with the addition of a horizontal bar, known as a tittle, or tilde. And though printers commonly cast this barred abbreviation as a single character, it was the rushed pens of scribes that eventually produced the symbol’s modern form: hurriedly dashed off again and again, the barred “lb” mutated into the abstract #.

Which is why the hashtag is often referred to as the pound sign. Keith also explains the origins of the Pilcrow (¶), the Ampersand (&), the Manicule (☞), and the Diple (>). Great article.