In reference to girls a comfortable medium where they are not a fat ass nor skinny, nasty bitch. Can be identified by their thunder thighs, muffin top, decent-to-plus-size titties, accented by very large posterior (ass). Commonly referred to as pleasantly plump, voluptuous, curvaceous.
"Hart" is in reference to a family from Baker County, GA where the boys of the family tend to seek after this species of fine women
Jesse: Did you see that girl?
Brady: Yea did you see her thunder thighs? She is definitely a woman who deserves Hart Thick status