Wharton, Law Lexicon , 1864, states that this is "a certain rent of money, coin, or other thing, anciently paid to persons upon or near the borders, who were men of influence, and allied with certain robbers and brigands for protection from the devastations of the latter; rendered illegal by 43 Eliz. c. 13. Also rent paid in cattle, otherwise called neat-gild.
See Also: Black Mail
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)