Green’s Dictionary of Slang

a poor quality pickled pig’s head.

1873		Sl. Dict. 202: Jerry Lynch, a pig’s head pickled. Term usually applied to the long Irish heads which are sent over here for sale in the poorer districts of London, and which are vastly different from the heads of ‘dairy-fed’ porkers.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.