Blackmail derives from the old practice of clan chieftains who ran protection rackets against farmers in the Scottish-English border counties. If the farmers did not pay the mail, the chiefs would steal their crops and cattle. This sense of mail is from Old English meaning rent or tribute and ultimately comes from the Old Icelandic mál, meaning speech or agreement. (This is one of those Old English words introduced by Viking raiders.) This sense is unrelated to other senses of mail and is now obsolete except for its use in blackmail.
Copyright 1997-2017, by David Wilton