1798, in a letter of Jane Austen, baby-talk form of little. Related: itty-bitty (1855); tiddy-itty (1852).
Examples from the Web for itty
Hey, little man, get on in here before you freeze your itty bitty butt.
I flatter myself that itty Dordy will not forget me at least under a week.
Itty said, “Well, I would like to see how, Since you already did- there are two up there now!”FreeChildrenStories.com Collection|Daniel Errico
The suspected representative of the “Guggenheimers” whined plaintively: “Itty tootsies awfy cold!”The Man from the Bitter Roots|Caroline Lockhart
Lock it up, papa; doin' to teep it for my itty dirl, when I get big, jess like mamma.Great Mysteries and Little Plagues|John Neal
My dear itty Dordy's remembrance of me is very pleasing to me—foolishly pleasing, because I know it will be over so soon.