In English-speaking culture, a milkman joke is a class of joke exploiting fear of adultery and mistaken paternity. 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 (at which time empty bottles were also collected). Men were commonly the main financial supporters of their families, and a man's wife tended to remain at home to care for their children and home. 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.
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Similar jokes referring to other professions, such as postmen, plumbers, pizza delivery drivers, and swimming pool cleaners, are also known.
Oftentimes, a different profession will replace the milkman in various jokes. For instance, in Spain, 'milkman' jokes are told about butaneros: persons who deliver butane, bottled fuel gas.
Example
Sam pulled up a stool at his favorite bar and announced, "My wife must love me more than any woman has ever loved any man!"
"What makes you say that?" the bartender inquired.
"Last week," Sam explained, "I had to take a couple of sick days from work. She was so thrilled to have me around that every time the milkman or the mailman came by, she'd run down the driveway waving her arms and hollering, 'my husband's home! My husband's home!'"