Green’s Dictionary of Slang

loaf (of bread) n.

[rhy. sl.; note twopenny n.1 suggests earlier usage]

a head, esp. brains, intelligence.

[UK]‘Doss Chiderdoss’ ‘The Hat Tonic’ Sporting Times 7 Feb. 1/3: Look what’s on her loaf of bread.
[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 145: Loaf: Head.
[UK](con. 1900s) in J.B. Booth Sporting Times 88: And the thought came into her loaf of bread / Just to pop in her I suppose.
[UK]D. Bolster Roll On My Twelve 134: Crash the Swede, Crash the Loaf.
[Aus]Argus *Melbourne) 15 Nov. 7s/2: You look slashing in your tit for tat (hat) on the back of your loaf of bread (head).
[UK]G. Kersh Fowlers End (2001) 131: To Bo-peep on their rolling billows [...] Where their loaves o’ bread repose.
[UK]S.T. Kendall Up the Frog 17: There I am wiv me loaf o’ bread on the weeping willow.
[UK]Wodehouse Much Obliged, Jeeves 129: I shook the loaf sadly.
[UK]A. Burgess 1985 (1980) 223: Sleep = get your head (swede, loaf) down.
[UK]S. Berkoff West in Decadence and Other Plays (1985) 137: Stay on at school / or work your loaf.
[UK]R. Walton ‘Cockney Jack’ [Internet] When Jack finished eating he put his this and that on his loaf of bread, picked up his Isabella, kissed his cheese and kisses, and left for work.
[UK]M. Coles Bible in Cockney 30: In three days you’re gonna ’ave your loaf cut off!

In phrases

use one’s loaf (v.)

to think, to act intelligently, to work things out; often as imper. use your loaf!

[UK]G. Kersh Night and the City 169: Well, you can use your loaf a bit.
[UK]G. Kersh They Die with Their Boots Clean 4: You don’t get yourself took prisoner by no Wog, not if you use your loaf you don’t.
[UK]J. Curtis Look Long Upon a Monkey 84: You could use your loaf.
[UK]C. MacInnes Mr Love and Justice (1964) 63: Naturally, boy! But do use your loaf!
[UK](con. WWII) B. Aldiss Soldier Erect 79: You want to use your bloody loaf [...] or we’ll never win the war the way you’re carrying on .
[UK]J. Sullivan ‘Watching the Girls Go By’ Only Fools and Horses [TV script] Rodney, use your loaf, you’re never gona pull a tart dressed up like Bertie Bassett?
[UK]P. Theroux Kowloon Tong 143: How many sorts are there, Bunt? Use your loaf.
[UK]M. Coles Bible in Cockney 69: So, you use your loaf and pour the new rise-and-shine into fresh wineskins.