This is a derivation of "hula bach" girl. When hola hoops first took off, Bach was going through a resurgence since a one-hit wonder had remixed one of his songs. One popular way of doing hula contests was to play Bach on a record player, spinning it faster and faster. (It was easy in those days to adjust the motor.)
"Hula Bach" girls had a tendency to gyrate a lot, and older folks considered it sinful and easy. "Hula Bach girl" was taken on as an ironic phrase by the girls. These days it refers to someone who is unashamed of their promiscuity.
Her friends thought she was an easy lay, but Donna knew she was a true holla bach girl.