Origin of word blackmail?

The original meaning of blackmail was not directly connected to the concept of extortion

The term appears to have originated in the Borders area between England and Scotland. Scottish lords would offer those without the protection of castles and chainmail a security against marauding Sassenachs. 

Given that Scottish lords were not above a bit of pillaging themselves, there was a subtext. This ran along the lines of nice little tenant farm you have here. Would be a shame if those bad guys were to come along and burn down your cottage.

Blackmail and crime

By the 19th century the connection between blackmail and criminal enterprise was firmly established, but more focussed around protecting social reputation rather than personal or property security. In 1840 the word entered the Oxford English Dictionary with the initial definition of  'payment extorted by intimidation or pressure'. This remains the legal standard, with 'payment' interpreted in broad terms.

A more metaphorical use of the term has also come into common use, often under the label 'emotional blackmail'. This usage is more difficult to pin down, particularly when applied to legal cases.

Etymology

Theories about the etymology of the blackmail are speculative. That it derives from two Scottish Gaelic words blathaich - to protect; and mal - tribute or payment seems plausible.

Blackmail in literature and film