That's part of the classic formulation, which dates to the 1960s, and which I first encountered in the 1980s in print. It's effectively a "consultant's matrix' of "known" and "unknown", giving a matrix of four elements: {KK, KU, UK, UU}.

The notion of unknown knowns strongly resembles the apocryphal Mark Twain quote; "It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so". (There's no apparent proof he'd said or written this.)

https://quoteinvestigator.com/2018/11/18/know-trouble/