Toronto. The first successful photographic processes used a weak salt solution as a fixer. Earlier experiments showed silver nitrate solutions reacted to light and could record scenes, BUT the silver nitrates quietly turned the whole medium dark. There was no way to turn off the silver salt (transparent) to metallic silver (opaque) process!
Shortly after the 1839 photographic processes were announced, Sir John Herschel suggested using Hyposulphite of Soda (Hypo) as a fixer. Sir John had discovered its property two decades earlier! The actual chemical turned out to be sodium thiosulphate, but the name HYPO stuck for many, many decades. When I turned to photography in the 1950s, the terms hypo and fixer were interchangeable and in common use.
Hypo or fixer had one simple job to do: wash out the remaining silver halides in the medium so they could not slowly convert to metallic silver and destroy the image. A lengthy water wash in turn removed the fixer.