Green’s Dictionary of Slang

pound of lead n.

also pound o’ lead

[rhy. sl.]

the head.

1960		J. Franklyn Dict. Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1984		Partridge DSUE (8th edn) 917/1: late C.19–earlier 20.