Green’s Dictionary of Slang

brown hat n.

a cat.

1960		J. Franklyn Dict. of Rhy. Sl.
1981		P. Wright Cockney Dialect and Sl. 104: brahn ’at ‘cat’.
1999		R. Walton ‘Cockney Jack and the Beanstalk’ 🌐 Jack was a very curious kid. He hadn’t yet heard how the brown hat died.