First of all, we are not talking about the Italian automobile manufacturer, but the much older meaning of the word that is derived from the Latin word fiat.

In the context of money as a currency, FIAT meaning can be translated as Determination by Authority, which in this case, the Government entity that regulates, determines, and maintains the value of money as a currency that otherwise has no intrinsic value or any other real use in society by masses.

And/or the value that is determined upon agreement by exchanges, regulators, or money policymakers.

Also, Merriam Webster describes the word Fiat as a command or act of will that creates something without or as if without further effort.