Definition of Fifteenths
In old English law, this was originally a tax or tribute, levied at intervals by act of parliament, consisting of one-fifteenth of all the movable property of the subject or personalty in every city, township, and borough. Under Edward III, the taxable property was assessed, and the value of its fifteenth part (then about £29,000) was recorded in the exchequer, whence the tax, levied on that valuation, continued to be called a "fifteenth,” although as the wealth of the kingdom increased, the name ceased to be an accurate designation of the proportion of the tax to the value taxed. 1 Bl.Comm. 309.
That's the definition of Fifteenths in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.