'belly-side' originates from Old English, specifically the word 'belg,' where 'belg' meant 'bag' or 'pouch,' referring to the stomach or abdomen.

'belg' transformed into the Middle English word 'bely,' and eventually became the modern English word 'belly.' The term 'belly-side' was formed by combining 'belly' with 'side' to describe the abdominal side of an organism.

Initially, it meant 'the side of the body where the stomach is located,' and this meaning has largely remained the same in modern usage.