"sweetheart, pretty girl," colloquial American English, 1921, perhaps a corruption of potato (c.f. sweet potato). Sweet patootie is recorded from 1919 as a generic exclamation.
Examples from the Web for patootie
Historical Examples of patootie
At Sixty-sixth he came sasshaying right down the car and said 'Hello, patootie!'Jill the Reckless
P. G. (Pelham Grenville) Wodehouse