Derives from the legend John Shenton. Especially used when you want to have sex with a bitch. But it can also be used if you are feeling really tired or drunk.
"Shit... I need to shentonise tonight bruv...'
"Are you going to shentonise her then?"
"Mate I wasn't up to that kinda shentonising last night... I'm just shentonised from all that work..."