Dorus received his nickname "Grandpa" (Dutch:"Opa") while still a young man. He had married Neeltje Huisman, a fisherman's widow who already had 6 children. Shortly after the marriage, the oldest of Neeltje's daughters had a child of her own, and so at only 23 years old Dorus became known as "Opa" in Den Helder where he lived. Although the nickname began as a joke, Dorus soon started acting and looking like a grandpa, and from that time on he became primarily known by his nickname.