Keep it simple stupid

KISS is an acronym for "Keep it simple stupid".

Stupid does not refer to the user.

The acronym was reportedly coined by Kelly Johnson, lead engineer at the Lockheed Skunk Works (creators of the Lockheed U-2 and SR-71 Blackbird spy planes, among many others). However, a variant "Keep it Short and Simple" is attested from a 1938 issue of the Minneapolis Star.

While popular usage has transcribed it for decades as "Keep it simple, stupid", Johnson transcribed it simply as "Keep it simple stupid" without a comma.

The principle is best exemplified by the story of Johnson handing a team of design engineers a handful of tools, with the challenge that the jet aircraft they were designing must be repairable by an average mechanic in the field under combat conditions with only these tools.

Hence, the "stupid" refers to the relationship between the way things break and the sophistication available to repair them.