Use your loaf

Rob: Hello and welcome to The English We Speak with me Rob and a very hungry-looking Li.
Li: (mouth full of food) Hello.
Rob: Erm, Li, you're talking with your mouth full. Can you wait till lunchtime please?
Li: No I can't—I'm starving.
Rob: Well, be careful—you're making such a mess in here.
Li: Ever since I started jogging before work, I just have to eat a big snack mid-morning or I feel faint by lunchtime.
Rob: Well, maybe, but you know you're not supposed to eat food in the studio. It's a really bad idea—you could damage the equipment. Oh no—look! There's salad and mayonnaise all over the recording desk! Li, use your loaf!
Li: Use my loaf? Yes, that's a good idea. If I use bread, I can make a sandwich, which will be much tidier.
Rob: No, I mean use your brain—think about things before you do them!
Li: Eh?
Rob: Well, in English, if you are slightly annoyed with someone because they are doing something they haven't thought about properly, you can say 'use your loaf'.
Li: Oh I see. Loaf here is slang for head.
Rob: That's right. It's a little old-fashioned but it's still something you might hear. My parents or grandparents would use it for when I did something a bit ... stupid.
Li: Here are some examples.
Old man:Use your loaf, son. Don't ride your bike on the motorway at night without lights. You'll get yourself killed.
Li: So, 'use your loaf' means think about things before you do them otherwise you will make silly mistakes.
Rob: That's right. So Li, use your loaf and stop eating food in the studio.
Li: Yes, next time I'll use my loaf and eat sandwiches.
Rob: No, don't eat food of any kind in the studio—not even sandwiches.
Li: But sandwiches are nice and tidy.
Rob: No they're not. You'll get lots of little crumbs everywhere.
Li: How about an apple?
Rob: No! Juice might drip!
Li: A banana?
Rob: No Li ... oh goodness!