use your head

Từ điển Oxford Advanced Learner 8th

use your ˈhead idiom
(BrE also use your ˈloaf) (informal)used to tell sb to think about sth, especially when they have asked for your opinion or said sth stupid
•‘Why don't you want to see him again?’ ‘Oh, use your head!’ From rhyming slang, in which loaf of bread stands for ‘head’.
Main entry: ↑use_idiom