use (one's) loaf

slang To think logically, rationally, or with common sense; to use one's head. The phrase comes from rhyming slang in which "loaf" is short for "loaf of bread," which rhymes with "head." 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 loaf

use your common sense. British informal

This expression probably comes from loaf of bread , rhyming slang for ‘head’.