An exclamation for something surprising. It is often used during the TV show "Scooby-Doo." It was mainly used to signify something shocking or for the discovery of a clue.
It's not a breakfast cereal, although it should be.
Johnny Bravo: "Jinkies!"
Ghostly Gardener: "GRRAAAHHHHHH!!!"
Johnny Bravo: "Jinkies... Jinkies... Isn't that a breakfast cereal or somethin'?"