"Merlin's beard, keep it down!"
-- Hagrid's reaction to Hermione's scream (OP20)
“Merlin’s Beard!” is a mild oath used to express surprise or exasperation – and is a form of swearing exclusive to the wizarding world.
It appeared most frequently during the events in OP. Arthur Weasley seemed quite fond of the phrase (OP7, OP9, OP24), and it was also spoken by Hagrid (OP20), Albus Dumbledore (OP27) and Cornelius Fudge (OP36).
In HBP, Cornelius Fudge likely confused the Muggle Prime Minister when he included it their conversation about the then current crisis (HBP1). Professor Slughorn said it when he encountered Harry in the grounds of Hogwarts (HBP22) and also when surprised by Tom Riddle’s questions about horcruxes in the past (HBP23).
Notes and interesting facts
As Merlin is such a famous and iconic wizard, his name is naturally invoked in other phrases.
Where a Muggle might possibly use "What in God's name...." or "What in hell":
- Ron says "What in the name of Merlin are you doing?" to Hermione while she is setting out concealed knitted hats for the house-elves (OP13).
- Ron also wonders "What in the name of Merlin was that about?" when Neville Longbottom tries to attack Draco Malfoy (OP17).
- There's obviously a missing word at the end of this one from DH6:
"And are they getting married in my bedroom?" asked Ron furiously. "No! So why in the name of Merlin's saggy left –"
- The British Envoy at MACUSA asks Newt Scamander "What in the name of Merlin are you doing in New York?" (WFT).
- Dumbledore asks Newt about the returned pendant containing his and Grindelwald's blood: "How in the name of Merlin did you manage to get . . . ?"(CG).
Additionally to the above, there are other attributes of Merlin to swear by besides his beard - including, apparently, his underwear:
"How in the name of Merlin's pants have you managed to get your hands on those Horcrux books?" (DH6)
"Snape, Headmaster! Snape in Dumbledore's study – Merlin's pants!" she shrieked, making both Harry and Ron jump. She leapt up from the table and hurtled from the room, shouting as she went, "I'll be back in a minute!"
"'Merlin’s pants'?" repeated Ron, looking amused. "She must be upset." (DH12)
"And what in the name of Merlin’s most baggy Y-fronts was that about?" (DH12)
Merlin is the wizard who aided and supported King Arthur (Wikipedia).
Regarding the first occurrence, although the date printed on the fourth Daily Prophet Newsletter (DP4) is 1 October 1999, the timeframe for those events was 1992-93. GF was not published until July 2000 and covered the school year of 1994-95.
From the Web
Harry Potter Wiki: Merlin's Beard
Writing by J K Rowling on WizardingWorld (Pottermore):