Tobie said his son's unmentionables, which he wears proudly around his parents' Pretoria home when he visits, are now close to eclipsing the flowing blond locks that have become "iconic".
"His hair was his trademark long before the underpants made an appearance. I am a conservative guy so for a while it used to bother me, but we see too little of him to worry about something so trivial. It does nothing for his rugby and doesn't change who he is as a person," said Tobie.
Faf's mother, Corrie, said she loves his hair. "I have always liked his hair long, and before school I kept it long at the back like MacGyver," she said.
There is no rigorous haircare regime or shelf full of conditioners and treatments, she said, just shampoo and water.
"He doesn't spend hours in front of the mirror faffing with his hair, he just washes it and blow-dries it and then he's off," she told the Sunday Times.
"It would be different if it wasn't clean, but the truth is he could have no hair and I would still love him."
Corrie said it was hard to watch her boy - at just 1.72m tall and weighing in at 88kg - brace for crushing tackles on the field.
"I never stop stressing about him because he thinks nothing of taking on the biggest men on the field. He plays with so much courage and confidence, but he always has had to," she said.
World Cup Special
'I was just happy he wasn't naked,' Faf de Klerk's dad on those famous undies
When Faf de Klerk shook hands with Prince Harry wearing only undies emblazoned with the South African flag, his father, Tobie, heaved a sigh of relief.
"I was just happy that he wasn't naked," he said this week.
The pint-sized scrumhalf is known for putting his body on the line - throwing himself at men far larger than him - but it is now his patriotic underpants that have been thrust into the limelight.
"If you go into a changeroom you are lucky to find people with any clothes," his father said. "Those underpants are his talisman, he wears them for every game. Some other guys were also in their undies, but they didn't go and greet Harry. It just shows you how confident the boy is."
"It's very frustrating to hear what has been said about him, especially when you know what the game plan was. People don't understand that you can't just go onto the field and do your own thing and still get selected," said Tobie.
He said the often vitriolic criticism came from a place of concern, because people loved the team.
"He has a very strong mind and he shrugs it off so we can do the same," he said.
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The proud parents said Faf's homecoming was low key. They shared a quiet braai with their son on the night he returned, before he left with the team on their countrywide victory tour.
"All he wanted was a braai, and he joined us at home. We hardly ever spend time with him and next week he is back in England for his club," Corrie said.
"But we're going to visit him in December."
Tobie said his son's unmentionables, which he wears proudly around his parents' Pretoria home when he visits, are now close to eclipsing the flowing blond locks that have become "iconic".
"His hair was his trademark long before the underpants made an appearance. I am a conservative guy so for a while it used to bother me, but we see too little of him to worry about something so trivial. It does nothing for his rugby and doesn't change who he is as a person," said Tobie.
Faf's mother, Corrie, said she loves his hair. "I have always liked his hair long, and before school I kept it long at the back like MacGyver," she said.
There is no rigorous haircare regime or shelf full of conditioners and treatments, she said, just shampoo and water.
"He doesn't spend hours in front of the mirror faffing with his hair, he just washes it and blow-dries it and then he's off," she told the Sunday Times.
"It would be different if it wasn't clean, but the truth is he could have no hair and I would still love him."
Corrie said it was hard to watch her boy - at just 1.72m tall and weighing in at 88kg - brace for crushing tackles on the field.
"I never stop stressing about him because he thinks nothing of taking on the biggest men on the field. He plays with so much courage and confidence, but he always has had to," she said.
"From the time he started playing rugby, he has always been told that he was too small to ever make a success of himself. He knew that so he would make sure he brought down the biggest player on the field, and look where he is now," she said.
"I spoke to him every day when he was in Japan, asking him what he had for lunch and who he was sharing a room with."
Before the Boks' march to victory over England in the final last week, the nippy scrumhalf had come in for caustic public criticism over his style of play and was rebuked for the kicking game he was tasked with implementing.
With this in mind, the De Klerks invested in a large TV and resolved to stay out of public places when watching their son play, his father said.