generally a woman, used to describe someone who has acne so bad on their back it looks like they were dragged through gravel
Ryan: Man, you see that girls face?
Me: Yeah? it's nasty
Ryan: Well, i heard that she's got it all over her back and body.
Jesse: Yeah, she's such a gravel back it's not even funny man.
Generally a FEMALE (not always)who has "gravel" embedded into her back from countless romps without so much as a blanket placed beneath her.
Were you talking to that girl? Yup. you need to run for the hills, she's a serious "gravel back" she's had more meat in her than Georgia smoke house.