The song is sung by a girl whose boyfriend went away but is now coming back. While he was away, another boy wanted to get into her pants, but she rebuffed him, and now he is spreading lies about her. However, the girl clearly has her big strong boyfriend wrapped around her little finger, and that other boy is in big trouble now that the boyfriend is back.
Now he's back and things'll be fineI find this whole thing to be very femdom (or is that just me?)
You're 'gonna be sorry you were ever born
Cause he's kinda big and he's awful strong
Hey he knows I wasn't cheatin'!
Now you're 'gonna get a beatin'!
What made you think he'd believe all your lies?
You're a big man now but he'll cut you down to size
My boyfriend's back he's gonna save my reputation
If I were you I'd take a permanent vacation
I picture the boyfriend as kind of big and dumb, but exceptionally devoted to his girlfriend. She will send him out to find the boy and forcefully bring him to her. I imagine the boy might get a bit beaten up in this process. Maybe punched once or twice. She will have him stripped and held bent over as she takes a cane and a belt to this boy's ass, saying "Now you're 'gonna get a beatin'!" And it will be a very severe beating, enough to leave his ass red and welted.
Then she'll put on a strap-on, and while her boyfriend watches teach this lying boy not to say untrue things about her. They will let it be known around the neighbourhood exactly what was done to him.
Well, that's how I hear the song anyways!