I remember him being mocked for this, but it always made complete sense to me.

Rumsfeld’s statement brought attention to the concepts of known knowns, known unknowns, and unknown unknowns, but these were in common use in US defense procurement by the late 1960s. In a 1968 study sponsored by the Aerospace Industries Association, Hudson Drake from North American Rockwell argued that defence contractors had to solve both known unknowns and “unanticipated unknowns”.[5] Also in 1968, Lt. Gen. William B. Bunker noted that when developing complex weapons systems “there are two kinds of technical problems: there are the known unknowns, and the unknown unknowns.”[6] The usage was common enough for an industry shorthand to have developed where unknown-unknowns were referred to as “unk-unks”.[7]

The term was commonly used inside NASA.[8] Rumsfeld cited NASA administrator William Graham in his memoir; he wrote that he had first heard “a variant of the phrase” from Graham when they served together on the Commission to Assess the Ballistic Missile Threat to the United States during the late 1990s.[9]