Brain Stomach is really bad stretch marks on a womans stomach after birth.
Man..... ! That girls brain stomach is so bad she's got to tuck it in like a shirt.
A stomach of a woman that has hundreds stretch marks, usually due to her being previously pregnant and/or fat, then losing weight, but keeping the stretch marks. Resembles the wrinkles in a person's brain. On her stomach.
Bob: "Why doesn't Jane ever wear a two-piece bathing suit?"
Johnny: "Because she has Brain Stomach."