basically a combination of smash and bang as a verb, to describe sex or the motion of sex. From my knowlege it came from a song "Lemme Smang it" by Yung Humma ft Flynt Flossy of Turquoise Jeep Records. The song is on youtube and Itunes. There is a specific dance which is also associated with word, as seen in the video.
that girl is fine, I just want to smang it to the moon.