Marx found the hidden engine of profit in the gap between what workers produce and what they earn.
The Old French surplus combined sur (over, from Latin super) and plus (more, from Latin plus).1 The word entered English in the fourteenth century to describe an excess, whatever remained after needs were met. In commerce, it meant the goods left over after a market had been supplied.
The agricultural surplus was the foundation of civilization itself. When farmers produced more than they consumed, the excess freed others to become soldiers, priests, artisans, and administrators. Surplus made specialization possible.
Karl Marx gave the word its sharpest economic edge. In Das Kapital (1867), he defined surplus value (Mehrwert) as the difference between the value a worker produces and the wage the worker receives. The gap, Marx argued, was the source of all profit under capitalism.2
The concept of surplus also shaped welfare economics. Consumer surplus, formalized by Alfred Marshall in the 1890s, described the difference between what a buyer is willing to pay and what they actually pay.3
In the mid-twentieth century, American military equipment left over from wartime was sold through surplus stores. The word that Marx had used to describe the extraction of labor's value became, in everyday American English, a label for cheap secondhand goods.4