It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tarsayati "dries up;" Avestan tarshu- "dry, solid;" Greek teresesthai "to become or be dry," tersainein "to make dry;" Latin torrere "dry up, parch," terra "earth, land;" Gothic þaursus "dry, barren," Old High German thurri, German dürr, Old English þyrre "dry;" Old English þurstig "thirsty."
1640s as both adjective ("disguised under an assumed name and character") and adverb ("unknown, with concealed identity"), from Italian incognito "unknown," especially in connection with traveling, from Latin incognitus "unknown, not investigated," from in- "not, opposite of, without" (see in- (1)) + cognitus, past participle of cognoscere "to get to know" (see cognizance). Also as a noun, "an unknown man" (1630s). Feminine form incognita was maintained through 19c. by those scrupulous about Latin. Incog was a common 18c. colloquial abbreviation.