Green’s Dictionary of Slang

poundcake n.

1. [1930s+] (US) an attractive woman.

2. [1940s+] (US gay) the buttocks.

3. [1970s+] (US gay) coprophilia.

4. [1990s+] (US) pornographic writing or magazines.

In phrases

make poundcake (v.)

[1980s+] (US gay) to have anal intercourse.