"There ain't no such thing as a free lunch" (alternatively, "There's no such thing as a free lunch" or other variants) is a popular adage communicating the idea that it is impossible to get something for nothing.
The initialisms TANSTAAFL, TNSTAAFL, and TINSTAAFL are frequently used. Uses of the phrase dating back to the 1930s and 1940s have been found, but the phrase's first appearance is unknown.
The "free lunch" in the saying refers to the nineteenth-century practice in American bars of offering a "free lunch" in order to entice drinking customers.
The phrase and the acronym are central to Robert Heinlein's 1966 science-fiction novel The Moon Is a Harsh Mistress, which helped popularize it.
The free-market economist Milton Friedman also popularized the phrase by using it as the title of a 1975 book, and it often appears in economics textbooks. Campbell McConnell writes that the idea is "at the core of economics".