Green’s Dictionary of Slang

(US) milk.

1887		True Northerner (Paw Paw, MI) 19 Oct. 7/1: ‘An the Jersey? What may that be?’ ‘Cream, you gillie! Don’t Jerseys give cream?’.
1908		Commercial (Union City, TN) 22 May 5/1: The woman ordered a cup of coffee with cream, two lumps of sugar [...] The man wanted a cup of coffee without cream [...] the waiter ordered, ‘Cup of mud, two chunks of ballast, milk the Jersey [...] Draw another in the dark’.
1947		B. Schulberg Harder They Fall (1971) 170: Tell Room Service to send up one Jersey.