A woman who is beginning her period without wearing a pad or tampon and leaves some red on her panties. Can also be used for a man: the act of pushing out a fart and drawing mud, leaving a small stain on your drawers. Similar to touching cloth, or skid marks.
Layne was priming his bowels for a huge dump when he ripped ass and inadvertently started spotting.
"I better get to the head, I think I'm spotting!"