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1.
This is normally associated with sexual intercourse carried out at a rough and vigorous pace (most commonly doggy-style), although it can be associated with any activity where it is performed with gusto. This means that you did whatever it was you were doing with particular enthusiasm and/or speed
Good morning Casper. I had a cracking time knocking the back out of Lady Clementine Smyth last night

or

Gadzooks, I could certainly knock the back out of a few glasses of Moet this evening
by Cockles August 29, 2006
 
2.
To do a chick doggie style and then all of a sudden pull a "wangthorpe" and sneak your johnson into her bunghole!
Back in 2004, I tried to knock the back out of Bev!
by shoesincan November 29, 2012