A Wise Acre
In the Middle Ages, the word "acra" referred to unoccupied land. During the Norman Conquest, it evolved into the word "acre" and designated the amount of land that a single ox could plow in one day. The visual image rightfully is a long, narrow tract of land, influenced strongly by the difficulty in turning a plough.
But, this definition was unfair, as the actual area of the tilled "acre" of land depended on the strength of the ox, the type of soil, levelness of the land, length of day, etc.
Thus, in the late 1200's, advisors to Edward I standardized an acre as a tract of land that was 40 rods long and 4 rods wide. Or, another attempt was to define it as "a selion of land sized at one furlong...long and one chain...wide." But, what are rods, selion, furlong, or chain?
The etymology of mathematical terms has always fascinated me...and open the doors to some interesting mathematical history. And, if you want to investigate the idea of an acre further, consider related variants: perch, lug, ox-goad, oxgang, virgate, or carucate?