The Spanish Conquistadores were making a map of their colonies (they owned all of America, South and North). They started drawing in the lines from down south - territory they knew well - and worked their way up.
Everything was going fine until they got to the New York area. "Hey, what's up there?" the map maker asked the governor, pointing to the vast emptiness above the Great Lakes.
The governor answered: "Here? (in Spanish: "Aca?"). Nothing (in Spanish: "Nada").
Hence the great blank emptiness became known as ACA-NADA, or in English, "There ain't nothin' here."