"No such thing as a free lunch" (alternatively, "There ain't no such thing as a free lunch", "There is 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 acronyms TANSTAAFL, TINSTAAFL, and TNSTAAFL are also used. The phrase was in use by the 1930s, but its first appearance is unknown. The "free lunch" in the saying refers to the formerly common practice in American bars of offering a "free lunch" in order to entice drinking customers.
The phrase and the acronym are central to Robert A. Heinlein's 1966 science-fiction novel The Moon Is a Harsh Mistress, which helped popularize it. The free-market economist Milton Friedman also increased its exposure and use, by paraphrasing it as the title of a 1975 book, and it is used in economics literature to describe opportunity cost. Campbell McConnell writes that the idea is "at the core of economics".
History and usage