Octothorpe is a word that came out of a phone company, and nobody is sure what half of it means.
The "octo" part is easy. The # symbol has eight points where its lines end, and octo is Latin for eight. The rest is a puzzle.
The word grew out of the touch-tone phone. World Wide Words says Bell Labs engineers developed touch-tone dialing in the early 1960s, adding the # key. The evidence suggests the name began as a joke among those engineers. The earliest printed use it knows of is from 1974. Today I Found Out points to a Bell Labs patent filed in 1973 that used the word. Merriam-Webster dates it to 1971, though Shady Characters notes that date has circular sourcing and no document from the 1960s backs any account.
Several people have been credited with it. Ralph Carlsen, writing in a 1995 Bell Labs memo, said an engineer named Don Macpherson coined it. Carlsen said Macpherson wanted a fresh, unambiguous name while he was instructing the Mayo Clinic on a new phone system.
The "thorpe" in that account is the athlete Jim Thorpe. Macpherson was part of a campaign to recover Thorpe's Olympic medals, which were taken away after the 1912 Stockholm Games because of earlier payments for playing baseball. They were returned in 1983. This is a good story, but it rests on one memo written decades later.
A rival account comes from Douglas Kerr. He wrote in 2006 that John Schaak and Herbert Uthlaut made up the term, first as "octatherp." Kerr said the "th" sound was included on purpose. World Wide Words notes that neither story has support from the time. Today I Found Out finds Kerr's version less plausible, since it is unclear why the spelling would change so quickly.
A third account comes from Wikipedia, which says Lauren Asplund claimed he and Howard Eby invented the word in 1964 as a joke. Kerr's 2014 follow-up concluded that Asplund likely did.
Other guesses exist. World Wide Words mentions a link to the Old Norse word thorp, "village," which it calls a stretch, and a dictionary link to James Oglethorpe, which it finds very unlikely.
The # has other names too. Americans often call it the pound sign, a name usually attributed to the Latin libra pondo and its "lb" abbreviation for pounds of weight.