What is "iron law of wages"

The iron law of wages is a proposed law of economics that asserts that real wages always tend, in the long run, toward the minimum wage necessary to sustain the life of the worker. The theory was first named by Ferdinand Lassalle in the mid-nineteenth century. Karl Marx and Friedrich Engels attribute the doctrine to Lassalle (notably in Marx's 1875 Critique of the Gotha Programme), the idea to Thomas Malthus's An Essay on the Principle of Population, and the terminology to Goethe's "great, eternal iron laws" in Das Göttliche.