The real name for this symbol is octothorpe. Rumor has it, it was coined by Bell telephone engineers who needed to name that other symbol on the telephone keypad. Since “octothorpe” sounds too much like an athletic octopus, people began referring to it as a “hash” sign. Probably because the slanted bars resemble the hash marks that designate rank on military uniforms.
Here in the US, we commonly referred to that symbol as a “pound” sign – which could get you into trouble in the UK, since a pound is something altogether different.