smash the back

A mans sexual actions on a woman, when he enters her vaginally or anally from behind in an over-excitable way.
The expression could be used best contextually in a shopping mall when your mates are cruisin' and he wants to describe ones finding of a bird across the way, he would like to "smash the back" out of, for the enjoyment of the conversation.

Oy brah... check out 3 o'clock, in line at Subway.. damnnnnn id be wise to smash the back out of that one .