Webster Dictionary(0.00 / 0 votes)Rate this definition: 1  Fiat(noun) an authoritative command or order to do something; an effectual decree. 2  Fiat(noun) a warrant of a judge for certain processes. 3  Fiat(noun) an authority for certain proceedings given by the Lord Chancellor's signature.
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Answers. Best Answer: Well, it depends what kind of dye you use. If the dye you are referring to is the semi-permanent kind available in boxes from the drugstore, you should be good about 2 weeks after you do the lice treatment.