it refers to the sexual activity between a man and a woman however, with a rather rough and fiesty edge to it.
Man 1 says to his friend - "Mate, i'm going to boomsmack the sh!t out of that girl tonight"
1. To burn; putting someone in check
2. When angry and swearing at someone
3. A sexual comment; to want to get frisky with someone
Eric: "Hey baby, can I get your number?"
Ashley: "Ummmm no! Didn't you go out with my friend last night?"
Nick: "Ohhh Eric! You just got boomsmacked!"
"I am sooo mad right now! I wanna boomsmack him!"
"Damn girl! I wanna boomsmack that ass!"