Blatant and unfair overcharging.
This isn't used to describe actual robberies - whatever time of day they might take place. It is a figurative phrase that associates an instance of unfair trading with actual robbery. Not just any old robbery, but one so unashamed and obvious that it is committed in broad daylight.
Daylight robbery really was the robbery of daylight.
It would be nice to locate the origin of this phrase, so let's go back to the 1690s. Like many English monarchs, William III was short of money, which he attempted to rectify by the introduction of the much-despised Window Tax. As the name suggests, this was a tax levied on the windows or window-like openings of a property. The details were much amended over time, but the tax was levied originally on all dwellings except cottages. The upper classes, having the largest houses, paid the most. Some wealthy individuals used their ability to pay as a mark of status and demonstrated their wealth by ostentatiously building homes with many windows.
What the Cavendish family, who owned Hardwick Hall (built 1590s), thought about it isn't recorded. On the one hand, they had cause for complaint - the property was famous for its light and airy interiors, as celebrated in the rhyme: "Hardwick Hall, more glass than wall". On the other hand, they were extremely rich and well able to pay.
Taxes are rarely popular, but the Window Tax, which was considered to tax the very stuff of life, that is, light and air, was singled out for particular loathing. People went to great pains to avoid paying it and many windows were bricked up for that reason. Many examples of buildings with brick window panels, sometimes with painted-on trompe l'oeil windows, still survive.
The sight of such windows is so much part of the English architectural folk memory that the example pictured, of a recently built property in Poundbury, Dorset, appears to have been built with fake bricked-up windows, even through the tax itself is long since abolished.
So, that's the case for the prosecution: the English were robbed of their daylight by the Window Tax. That's daylight robbery in anyone's book, so do we need to look any further for the origin of the phrase? Well, yes we do.
Let's move to the 20th century for the case for the defence. The phrase isn't known in print until 1916 in Hobson's Choice, a comic play by Harold Brighouse. Even there the context doesn't explicitly link it to unfair overcharging or the like. We have to wait until 1949 for a citation that is clearly related to a purchase, in Daniel Marcus Davin's Roads from Home:
"I can never afford it, said his sister. It's daylight robbery."
So, if the phrase came from the Window Tax, why no mention of it in print for over two hundred years after the tax was introduced?
If we are looking for evidence that is beyond reasonable doubt, the Window Tax story doesn't provide it. Unless and until evidence that relates the phrase to the tax is found we have to say that the origin is unknown.