A person who is on the verge of being fat. Eating one more biscuit would literally make the person fat. Picture a fat person full as hell with the last thing on a his plate, a biscuit, in hand huffing and puffing knowing he shouldn't eat it, but damn well will. That is a biscuit away.
Person A: Have you seen Jay's sister lately? She was fine back in the day wasn't she?
Person B: Yeah she was, but Russel Glasgow say she a biscuit away now.