Green’s Dictionary of Slang

pecnoster n.

[pecker n.² (2) + pun on SE paternoster, our father]

the penis.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 231: Rat, m. 1. The penis; ‘the pecnoster’.