Green’s Dictionary of Slang

1. fine, pleasing.

1943

R. Llewellyn None But the Lonely Heart 192: ‘You’re cake,’ he says.

2. easy, simple.

1967–8		Baker et al. CUSS 93: Cake course Easy course.
1983		Eble Campus Sl. Mar. 1: cake – easy, simple.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 315: Compared to what she’s been through, this thing should be cake.
1992		D. Burke Street Talk 2 15: This class is cake.
2013		B.H. Shepherd ‘Rogues Gallery’ in ThugLit July-Aug. [ebook] New guards always think Cellblock Thirteen is going to be a cake watch.