The word incognito was borrowed in the mid-seventeenth century from Italian, where it meant "unknown". The same was true in Latin as incognitus, which was composed of the prefix in-, meaning "not", and cognitus, the past participle of cognoscere, "to get to know" (therefore something incognito was "not known"). Cognoscere also has a prefix, con-, that meant "with", which leaves the root gnoscere, "to know" (also the etymon of noble, connoisseur, and cognition). In- traces to Proto-Indo-European en (also "not"), con- to PIE kom, meaning "along", and gnoscere to the Proto-Indo-European root gno, also "know" (this is the source of gnome, diagnosis, ignorant, and a lot of other cool words). Usage of the word incognito in literature over time has been trending slightly downward, but is generally pretty constant.
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