1630s, "authoritative sanction," from Latin fiat "let it be done" (used in the opening of Medieval Latin proclamations and commands), third person singular present subjunctive of fieri be done, become, come into existence," used as passive of facere "to make, do" (see factitious). Meaning "a decree, command, order" is from 1750. In English the word also sometimes is a reference to fiat lux "let there be light" in Gen. i:3.