A physique, while not overweight (and possibly underweight), lacks any visible lean, striated tissue.
The term is often NOT used or understood by the uneducated, untrained eye. To the uneducated, untrained eye, a skinny fat female may appear to have a physique of the same caliber as an individual who is comprised of significantly more lean tissue. The term is particularly relevant when discussing women at NASCAR events; the types that may be skinny but have no muscle to speak of. The term is equally important while eating at Denny’s and discussing how unappealing the waitress is.
Untrained eye: "Dude, that chick is so hot."
Trained eye: "Nah, Dogg, that's Amanda Schroeder; she's skinny fat as all hell."
Untrained eye: "What do you mean?"
Trained Eye: "Look at her arms."