to describe something so slippery and slick that it will slide off of any surface, or a person who is so untrustworthy and clearly greasy that they gleam in the sun.
Thomasville slicker is the act of using oil to lubricate the asshole of your girl. Once the asshole is effectively oiled up you take her to the nearest Thomasville with baseball bat in hand. Bend her over one of the beds in the Thomasville and proceed to buttfuck the shit out of her with a baseball bat until security comes and asks you to leave.
Joe: "Hey boss, how was your weekend?"
Mitch: "It was alright, me and my girl ran a few errands at the outlets. I ended up giving her a Thomasville Slicker though, which was pretty nice."