Green’s Dictionary of Slang

crust of bread n.

the head.

1960		J. Franklyn Dict. of Rhy. Sl.
1971		J. Jones Rhy. Cockney Sl.
1980		(con. 1930s) Barltrop & Wolveridge Muvver Tongue 29: Head: loaf or crust, short for loaf or crust of bread.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.