I once heard A L Lloyd introduce the 'goodnight' ballad (gallows valediction) 'The Robber', a.k.a. as 'Tedburn Hill', as follows:
Goodnight ballads were written by hack ballad-makers and sold at public hangings, sometimes with part of the proceedings going to the hanged man's family. They were called 'Goodnight ballads' in reference to the practice of the onlookers at the hanging calling 'goodnight' as the rope went taut around the hanged man's neck.
The explanation was said to come from Daniel Defoe, though I have never been able to find the reference.
Jim Carroll