Green’s Dictionary of Slang

(Irish) the act of grabbing someone by the testicles to inflict pain and humiliation.

1993		R. Doyle Paddy Clarke Ha Ha Ha 108: Pruning was banned in our school.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Pruning (v): when you get your testicles grabbed and squeezed hard usually by a few guys holding you down or sometimes suddenly by one bully!