Green’s Dictionary of Slang

anything simple or very easy.

1925		Gleason & Taber Is Zat So? II i: clint: You don’t think it would do Chick any harm, do you, Fitz, to have a little practice go? fitz: Harm? Cream in the can, old boy!
1995		J. Cameron Vinnie Got Blown Away 71: Sometimes it’s a cream, sometimes it’s a fuck-up.
1996		Eble Campus Sl. Sept. 2: cream – an easy course.