TANSTAAFL is an acronym that attempts to describe the cost of decision making and consumption. It stands for “There ain’t no such thing as a free lunch.” TANSTAAFL expresses the idea that even if something seems like it is free, there is always a cost, no matter how indirect or hidden.
As explained by Investopedia, in financial circles TANSTAAFL refers to the opportunity cost paid to make a decision. The decision to consume one product usually comes with the trade-off of giving up the consumption of something else.
This phrase is thought to have originated more than a century ago. Many saloons in the U.S. used to provide free lunches to their patrons, but required them to purchase drinks in order to get them.
Although the phrase is a double negative, it is not intended to be interpreted as such. Therefore, the alternate acronym TINSTAAFL (“There is no such thing as a free lunch”) is also used.