Hi Katlyn Roberts, sorry it was my error in my response to you as I was speed typing! The correct phrase is ‘cold enough to freeze the balls off a brass monkey’.
The brass monkey was a tray that held the cannonballs. Here’s an image and more in-depth explanation from a boater’s forum: https://www.modelboatmayhem.co.uk/forum/index.php?topic=11449.0
Have fun using it!