From Longman Dictionary of Contemporary Englishnot an ounce of fat (on somebody)not an ounce of fat (on somebody)if there is not an ounce of fat on someone, they are thin and usually look healthy → ounce
Examples from the Corpus
not an ounce of fat (on somebody)• Under their chestnut coats there was not an ounce of fat and their muscles moved without effort.• He was surprised, there was not an ounce of fat on him, but he had shed five pounds. 