Why is the ninth month called September?
1640s, from Italian incognito "unknown," especially in connection with traveling, from Latin incognitus "unknown," from in- "not, opposite of, without" (see in- (1)) + cognitus, past participle of cognoscere "to get to know" (see cognizance). Fem. form incognita was maintained through 19c. by those scrupulous about Latin. Incog was a common 18c. colloquial abbreviation.