It's originally a Marxist term, of course. Marx used it to imply that employees under a capitalist system who don't "own the means of production and distribution" are virtually powerless, not much different from slaves. The slavemasters in this metaphor are the capitalists -- defined as those who own "capital," the tools of production. Marx was writing from the viewpoint of nineteenth-century industrial society, when the tools of production were, literally, big iron -- machinery, railroads, and so forth.
Presumably Marx would have lumped most IPs today under his "petty bourgeoisie" label, neither wage slave nor capitalist, but mostly destined to be squeezed down into the wage-slave class. Of course, Marx never saw a computer, and never imagined a world in which intellectual capital generates massive wealth overnight while big iron rusts.