use your loaf
use (one's) loaf
To use one's own intelligence and intellectual ability; to think logically and rationally. In this phrase, "loaf" refers to one's head or brain. Often said as an imperative. Primarily heard in UK, Australia. Come on, Dean, I know you can figure this out on your own. Use your loaf! Jenny finally remembered to bring the right books home to do her homework. I'm glad she's finally using her loaf.
use your loafuse your common sense. British informal
This expression probably comes from loaf of bread , rhyming slang for ‘head’.