This Christmas custom just keeps naughtier each year.

A prankster mom shaved the word “Elf” in her son’s hair, then blamed it on the “Elf on the Shelf,” giving the boy’s younger sibling a Christmas conniption.

The troll-y tradition of hiding a pesky elf has become a holiday mainstay in recent years. The elf supposedly spies on children in the days leading up to Christmas, then reports back to Santa Claus on the kids’ behavior. Each year, parents seem to outdo themselves in the levels they’ll go to in punking their kids in the name of elf mischief.

But a Welsh mom of three, Amanda Johns, has perhaps gone a hair too far with her prank. She enlisted the help of her teenage son Iestyn, who was in on the joke. Iestyn pretended to be asleep as Amanda shaved the word “Elf” into his scalp. Then, she placed the clippers into the hands of their household elf — framing him for the crime.

When Amanda’s younger daughter, Tegan, woke up and discovered the wreckage in her brother’s hair, she was shocked.

“She runs into my room and asks, ‘Where is the elf?!’ but when she found him in Iestyn’s room with the clippers she was flabbergasted,” Amanda told Caters News Agency.

Amanda shared the prank on Facebook, where her friends and followers questioned her parenting judgment.

“They thought I had actually shaved his hair in his sleep, but I am not that cruel,” she said.

“[Iestyn] is the best big brother and will do anything to make his little sister smile — she looks forward to the Elf’s antics every morning.”

Amanda said she planned this elf antic out a week in advance, and asked her son not to cut his hair in preparation for the prank.

“It is completely worth it to see Tegan’s face light up in the morning,” she said.

In years past, she’s replaced all the family pictures in the house with pictures of elves, and given her son a makeover in his sleep — complete with drawn-on abs.

“When Tegan had a cold, I put a skeleton on the sofa, and she believed he caught the ‘elf flu’ and needed lots of chocolate to make him better,” Amanda said.

“It is great fun for all of us.”