film for your brownie, n. — Green’s Dictionary of Slang

film for your brownie n.

[pun on the Brownie (camera) + brownie n.¹ (4a)]

toilet paper.

1972		B. Rodgers Queens’ Vernacular.
1986–7		Maledicta IX 194: This article and series devoted to sexual slang would be incomplete without some notice of catch phrases, both British and American: […] film for your brownie (toilet paper).