No man boobs or muffin tops. A person who looks healthy and lean with a low body fat percentage. Body mass index is underweight (under 18.5) or healthy (18.5 to 24.9). A female who is 168cm tall, for example, and weighs 58kg would be considered skinny.
"She is so beautiful and skinny. Wish I was as skinny as her."
"Based on your height and weight, you are in the healthy BMI range and are nice and skinny."