1895, probably Southern U.S. dialect, first attested in Joel Chandler Harris, perhaps an alteration of precise (q.v.), or a merger of prim and sissy [OED]. Related: Prissily; prissiness.
["]Then Mrs Blue Hen rumpled up her feathers and got mad with herself, and went to setting. I reckon that's what you call it. I've heard some call it 'setting' and others 'sitting.' Once, when I was courting, I spoke of a sitting hen, but the young lady said I was too prissy for anything."
"What is prissy?" asked Sweetest Susan.
Mr. Rabbit shut his eyes and scratched his ear. Then he shook his head slowly.
"It's nothing but a girl's word," remarked Mrs. Meadows by way of explanation. "It means that somebody's trying hard to show off."
"I reckon that's so," said Mr. Rabbit, opening his eyes. He appeared to be much relieved.
[Joel Chandler Harris, "Mr. Rabbit at Home"]