Why do we call someone who continually takes the fall for someone else a “whipping boy”?

In the mid-seventeenth century, young princes and aristocrats were
sent off to school with a young servant who would attend classes and
receive an education while also attending to his master’s needs. If
the master found himself in trouble, the servant would take the pun-
ishment for him, even if it were a whipping. He was his master’s
“whipping boy.”