The origin of the phrase "happy as a sandboy" is unknown, but it is thought to have originated in 19th-century England. At that time, sandboys were young boys who sold sand from the street corners. They were known for their cheerful demeanor and willingness to work hard, even in difficult conditions. As a result, they became associated with happiness and contentment.
The phrase has since been used to describe anyone who is very happy and carefree. It is often used to describe children, but it can also be used to describe adults.