Ratchet is If Ghetto and Hot Shitty Mess had a baby, and that baby had no father, and became a stripper, then made a sex tape with an athlete, then became a reality star!
Basically Ratchet is a term for someone who is such a ghetto/slutty/ugly/trashy hot mess that you have to use a word that doesn't even technically apply because what you are seeing is such a mess that it goes beyond any normal description.
God Damn! Look at that Ratchet ass bitch over there!
You are pointing out an extremely unattractive woman walking by in a Bikini too small for her body type and her tampon string hanging out.