The acronym for “keep it simple, stupid” is attributed to Kelly Johnson, an engineer at the U.S. weapons company Lockheed. Although there are several other variations, the principle states that systems work best if they are kept simple.
Complexity should be avoided. Johnson had given a team of design engineers a set of tools, then challenged them to design a jet aircraft which can be repaired by an average mechanic under war conditions with these tools only.
There is nothing original about KISS, however. See the statement attributed to William of Ockham that “among competing hypotheses, the one that makes the fewest assumptions should be selected”; to Leonardo da Vinci that “simplicity is the ultimate sophistication”; to Mies van der Rohe that “less is more”; and to Antoine de Saint Exupéry “It seems that perfection is reached not when there is nothing left to add, but when there is nothing left to take away“.