Green’s Dictionary of Slang

(US) (to be) completely out of money.

1899		J. Flynt Tramping with Tramps 117: I’s been a moocher, an’ now I’s shatin’ on me uppers (I am broke).
1929		D. Hammett ‘Fly Paper’ Story Omnibus (1966) 44: I’ve been shatting on my uppers for a couple of months.
1930		‘Dean Stiff’ Milk and Honey Route 213: Shatting on one’s uppers – To be broke, penniless or clean.