Green’s Dictionary of Slang

packy n.¹

(W.I.) the skull, the head.

[1869		H.G. Murray ‘Tom Kittle’s Wake’ in D’Costa & Lalla Voices in Exile (1989) 96: A ‘packy’ or small calabash cup was held underneath, into which the blood trickled].
1943		cited in Cassidy & LePage Dict. Jam. Eng. (1980).

In phrases

open packy (v.)

(W.I.) to reveal one’s innermost thoughts.

1943

cited in Cassidy & LePage Dict. Jam. Eng. (1980).