back knacker

To be bollock deep in a female or grossly overworked.

"I was up to my back knacker in a filthy hoe".

"I'm up to my back knackers in it at present"

back knacker by L.B. Seed August 21, 2004

back door knockers 

Testicles, or bollocks - a name given due to the action of the guy's balls slapping a girl's arsehole during frontal sex

"I was really going for it and she was loving my back door knockers."

back door knockers by Bollywoody April 26, 2015