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" (from PIE root *bheue- "to be, exist, grow"), used as passive of facere "to make, do." 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 Genesis i.3.
Dixitque Deus: Fiat lux. Et facta est lux. [Vulgate]
*bheuə-, also *bheu-, Proto-Indo-European root meaning "to be, exist, grow."
It forms all or part of: Bauhaus; be; beam; Boer; bondage; boodle; boom (n.1) "long pole;" boor; booth; bound (adj.2) "ready to go;" bower; bowery; build; bumpkin; busk; bustle (v.) "be active;" byre; bylaw; Eisteddfod; Euphues; fiat; forebear; future; husband; imp; Monophysite; neighbor; neophyte; phyletic; phylo-; phylum; phylogeny; physic; physico-; physics; physio-; physique; -phyte; phyto-; symphysis.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhavah "becoming," bhavati "becomes, happens," bhumih "earth, world;" Greek phyein "to bring forth, make grow," phytos, phyton "a plant," physis "growth, nature," phylon "tribe, class, race," phyle "tribe, clan;" Old English beon "be, exist, come to be, become, happen;" Old Church Slavonic byti "be," Greek phu- "become," Old Irish bi'u "I am," Lithuanian būti "to be," Russian byt' "to be."
As "science teaching based on a fundamentalist interpretation of the Book of Genesis, the scientific theory attributing the origin of matter and life to immediate acts of God," opposed to evolutionism, it is attested from 1880. Century Dictionary (1897) defines creationism in this sense as "The doctrine that matter and all things were created, substantially as they now exist, by the fiat of an omnipotent Creator, and not gradually evolved or developed."
Creation science is attested by 1970 as an alternative name for a theory of science not inconsistent with Christian fundamentalism. Creationist (n.) in an "anti-Darwin" sense is attested by 1859 in a letter of Darwin's, and it is said to be used in Darwin's unpublished writings as far back as 1842.
James Ussher (1581-1656), Archbishop of Armagh, Primate of All Ireland, and Vice-Chancellor of Trinity College in Dublin was highly regarded in his day as a churchman and as a scholar. Of his many works, his treatise on chronology has proved the most durable. Based on an intricate correlation of Middle Eastern and Mediterranean histories and Holy writ, it was incorporated into an authorized version of the Bible printed in 1701, and thus came to be regarded with almost as much unquestioning reverence as the Bible itself. Having established the first day of creation as Sunday 23 October 4004 B.C. ... Ussher calculated the dates of other biblical events, concluding, for example, that Adam and Eve were driven from Paradise on Monday 10 November 4004 BC, and that the ark touched down on Mt Ararat on 5 May 1491 BC "on a Wednesday". [Craig, G.Y., and E.J. Jones, "A Geological Miscellany," Princeton University Press, 1982.]