No, that's not a typo; it's an acronym. That big jumble of tongue-twisting letters stands for "There Ain't No Such Thing as a Free Lunch," and this phrase represents a bite-sized version of the novel's views of economics and the government's role in it.
We're first introduced to TANSTAAFL when Stu and Mannie are in the taproom discussing Luna society. When Stu asks what TANSTAAFL means, Mannie tells him and then points to a sign across the room that says—you guessed it—FREE LUNCH. He notes the sign isn't being truthful "or these drinks would cost half as much." In other words, the taproom provides free food but makes up the difference by hiding the costs in its drink prices. Mannie adds that "anything free costs twice as much in the long run or turns out worthless" (11.99).
The idea returns later in the novel when the Chairman is trying to convince Mannie to take the job of Protector Pro Tem. The Chairman tells Mannie:
"Convince Loonies they could not win. Convince them that this new setup was to their advantage—emphasize benefits, free schools, free hospitals, free this and that—details later but an everywhere government just like on Terra." (19.52)
Free sounds good, right? But the novel has already warned us that there is no such thing as free, and in the end, the Loonies would have to pay for all those free services through taxes. They would also have to accept the "everywhere government," meaning they'd pay with their liberties and freedoms, too.
From bars to governments, then, TANSTAAFL serves as a handy reminder of the novel's economic philosophy.