In Victorian-era London, a tosher was originally someone who scavenged through garbage and filth to find items of value. This might include mudlarks, who searched the mud along the riverbank for valuable objects, the rag and bone men who searched through garbage piles, and innumerable other scavengers. But in 1851 Henry Mayhew published his massive work London labour and the London poor, in which he used the term to refer specifically to those who braved the sewers to look for those small and potentially valuable items that washed down the sewer drains of London. This usage caught on, and became the most common usage.
Mayhew was the first to document the occupation of the 'sewer-hunter' (his preferred term), and did so in great detail. He was surprised to note that hunting the sewers was a relatively profitable and respectable profession, and one that was not generally considered to be unhealthy, although it certainly carried its risks.
By his best estimate, there were around 200 men working as toshers at the time, estimated to average perhaps six shillings a day (admittedly, by their own report), which in today's money would be about $50. Of course, while the toshers did find coins, they were mostly collecting scrap -- bits of rope, scrap metal, rags, and anything else that could be sold. (This during a time when dog droppings and cigar butts were viable commodities -- there wasn't much that couldn't be sold in London).
"These 'Toshers' may be seen, especially on the Surrey side of the Thames, habited in long greasy velveteen coats, furnished with pockets of vast capacity, and their nether limbs encased in dirty canvas trousers, and any old slops of shoes... provide themselves, in addition, with a canvas apron, which they tie round them, and a dark lantern similar to a policeman’s; this they strap before them on the right breast, in such a manner that on removing the shade, the bull’s eye throws the light straight forward when they are in an erect position... but when they stoop, it throws the light directly under them so that they can distinctly see any object at their feet. They carry a bag on their back, and in their left hand a pole about seven or eight feet long, one one end of which there is a large iron hoe." -- Henry Mayhew, London labour and the London poor, Vol .2
There was easily enough sewer to support 200 hunters, with 360 major sewer lines and innumerable offshoots -- and London was a crowded city, with a population of 2,350,000 and growing fast. However, it was far from easy pickings, as Mayhew calculated that these lines averaged less than four feet in diameter, and the lower tunnels were flooded twice daily as the high tide washed up the Thames river, along with sluices that were opened at low tide allowing waste to flow in waves through the sewers.
Aside from the threat of drowning, there was also the possibility of tunnel collapses, poisonous gas build-up, and infected rat bites. There were also reports of rat swarms attacking and killing toshers, and perhaps less likely reports of feral hogs living in the sewers under Hampstead. After 1840 hunting in the sewers was made illegal, and a reward was offered to anyone who turned in a tosher, leading to toshing being primarily a night-time profession.
There is very little record of the downfall of the occupation of toshing, but it is likely that the toshers were not helped by the modernization of the London sewer system in the 1860s, under the supervision of Joseph Bazalgette. One part of this project was to move the sewer outlets well downstream of London, which most likely made the toshers 'commute' unrealistic.
The word tosher was also used in three other senses, and it is in fact only these other definitions that have made it into most dictionaries:
1. Tosher was documented in 1859 to refer to thieves along the Thames who stripped copper sheathing from the bottoms of ships or from dockyard stores.
2.In 1889 tosher was noted to be college slang for an 'unattached' student -- a non-collegiate student at a university having residential colleges.
3. And in 1885 it was documented in use to refer to a small fishing vessel that stayed close to the shore.
Further reading: Smithsonian Magazine: Quite Likely the Worst Job Ever
London labour and the London poor: a cyclopaedia of the condition and earnings of those that will work, those that cannot work and those that will not work, Volume 2, Henry Mayhew
Cass, 1851.