Gary Shirley should know better.
We know he's not the sharpest tool in the shed, but if there's one thing you don't do, it's call your baby mama "sluttish" on the phone ... to her cousin.
In a TMZ audio recording from just last month, Gary is on the phone with Krystal, the cousin of Teen Mom star / Gary's baby mama Amber Portwood ...
Gary explains to Krystal that he kind of has a crush on her, pointing out, "You've got things that Amber don't." Like a job, and/or a full set of marbles.
Then Shirley drops an all-time quote, praising Krystal because: "Amber is sluttish. You've been with six dudes, Amber's been with f**king a million."
Sums it up well. Listen above.