it means they are dancing together; with the girls butt or crotch rubbing against the guys butt or crotch.
So they have something to hold onto!
Well if you go to your local night club you will see that girls with low-cut shirts and tight jeans love grinding guys if hopes that they will go home with them and maybe put the guy will put his...
Yeah.....why do you think they do it. It makes penetration deeper and what not. Its not rocket science....
Grinding is dancing with a girl with close physical contact between your hips/legs and her arse/hips. Could including kissing, groping etc
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