This is a term used when an offer or deal etc. is very unfair to one side... in this situation they might exclaim that it is like daylight robbery, since the price is so high that they are blatantly robbing them in such a way as they should not believe that they can get away with it.

This term may have been introduced during the days of the window tax in Britain. During this period, many people could not afford to pay the tax and so bricked over their windows (in fact many houses in London still have them bricked over), meaning that the government had stolen their daylight, a thing that they shouldn't expect to get away with, in effect a case of daylight robbery.