Definition of Forty
In land laws and conveyancing, in those regions where grants, transfers, and deeds are made with reference to the subdivisions of the government survey, this term means forty acres of land in the form of a square, being the tract obtained by quartering a section of land (640 acres) and again quartering one of the quarters.
That's the definition of Forty in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.