Green’s Dictionary of Slang

1. (UK juv.) a lump of spit [yock v. (2)].

1975		A. Bleasdale Scully 14: This teacher [...] she kept hitting me with the board duster because someone put yocker on her back. But it was Ryan [...] what spat at her.
2003		N. Griffiths Stump 13: Shurrup, will yeh. Yer coveein me in fuckin yocker.

2. (Irish) a testicle [in ext. sense of ‘lump’].

2001

G. Coughlan Everyday Eng. and Sl. 🌐 Yockers (n): Balls.