[1][better source needed] This class of jokes has its roots in the early part of the 20th century, prior to the regular availability of milk in supermarkets.
At that time, milk in glass bottles was delivered directly to customers' houses by milkmen, generally in the morning.
Typically, men were the main financial supporters of their families, while women tended to remain at home to care for their children and house.
As the milkman would visit the home at a time when the husband would be away at work, this created an opportune situation for adultery.
Similar jokes referring to other professions, such as postmen, plumbers, pizza delivery drivers, and swimming pool cleaners, are also known.