Joan Gerbereux recently sent this intriguing question to me:
“Would you discuss hash tag? I don’t know if it’s one or two words, what it means, or how the ‘pound sign’ became something else. Do you?”
It is a word with the symbol # in front of it, used especially in microblogging to identify or search for subjects or on Twitter to denote the subject of a post. It is a blend of hash sign and tag. Hast sign is probably a mispronunciation of hatch sign. To hatch something is to score it or engrave lines in it. The meaning probably originated in “crosshatch”.
I’m not sure when the pound sign or number sign became a hash sign or mark. A hash-mark during World War II meant the symbol of rank, worn on the left sleeve of enlisted men. This sense arose in the 40s.
This symbol (#) was called the pound sign only in the US; £ was the pound sign elsewhere. It emerged in the 20s but didn’t last long in general speech (or writing). Can you remember seeing 10# standing for “10 pounds”? It seems to have become a specialized technical symbol.