This statement is usually used as verbal sexual harassment for the purpose of an attempt at seduction. The statement implies lending the speaker the use of one's ass, presumably for sex. In a sentence, this is most effective when shouting from a distance, such as from a moving car, or the second story of a building.
Four men are riding in a car. One see an attractive female, but knows he cannot actually seduce her for reals, so he decides to shout out to window at her. His statement of choice is "Gurrrrl, let me get some booty up off you!"
As the girl turns, he receives some self-satisfaction.