Blackmail is an act, in some jurisdictions a criminal offence, involving threats to cause loss or damage to another unless a demand is met. Essentially, it is coercion, or extortion, involving threats to reveal either true or false information about a person to the public, a family member, or associates. It can also mean threats of physical harm.
That is its current meaning. However, before 1774, it meant “protection money,” an amount paid to safeguard tangible items, such as houses, cattle and one’s physical person.
“Blackmail” entered the language in 1530, when it described the practice among English farmers living along the border to give money or goods to Scottish brigands to protect themselves and their property from pillage.
Sixteenth-century Scottish farmers paid their rent, or mail, to English absentee landlords in the form of white mail, silver money, or black mail, rent in the form of livestock or produce. The term black mail took on a bad connotation only when greedy landlords forced cashless tenants to pay much more in goods than they would have paid in silver. Later, when freebooters along the border demanded payment for free passage and “protection,” the poor farmers called this illegal extortion black mail, too.
The “mail” part of this word has nothing to do with letters or parcels delivered by the postman. It derives from Middle English “male,” meaning rent, tribute, or an agreement. This tribute was paid in goods or labour. “Black” comes from our habit of associating that color with evil or underhanded doings (black market, black-hearted, blacklist, black arts, black magic).
Alternatively, it may be derived from two Scottish Gaelic words “blathaich” pronounced (the th silent) bla-ich (to protect) and mal (tribute, payment).
Although the word “blackmail” has only been in use since the sixteenth century, but because it’s all about money, I suspect that the practice has been going on since humans began to own property, whether it was a superior stone knife or a particularly succulent kill. And it prospers in schools (and other places) today in the form of bullying.