Lyric discussion by ihavechappedlips 

Cover art for I Woke Up In A Car lyrics by Something Corporate

I think the Mississippi line is literal. They could have stopped by the Mississippi River and went in. I imagine you would get pretty dirty in the mississippi. She probably still had clothes on, and it was probably in the summer so she would've had shorts... and so he saw that her knees were still dirty.

Yeah.. that's all I've got