-- Hagrid (CC1.8)
“Scarramanger” is the term that Vernon Dursley uses, during one of the adult Harry’s dreams about his past, to describe Rubeus Hagrid, when Hagrid finally catches up with them at the Hut-on-the-Rock (CC1.8).
Harry's Uncle Vernon thinks he can get the better of Hagrid and tells his family to hide behind him while he deals with the intruder.
While no dictionary definition of "scarramanger" exists, it may be related to the word scaremonger, someone who "spreads frightening or ominous reports or rumours" (Oxford Dictionaries).
It is also interesting to note the similarity in sound and spelling to Scaramouch, a stock character in Italian farce. The word can therefore also refer to a "cowardly buffoon" or "a rascal or scamp" (Merriam-Webster, Wikipedia).
Scaramanga is an evil character from the James Bond book/film "The Man with the Golden Gun" by Ian Fleming (Wikipedia).
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016: Harry Potter and the Cursed Child, Parts One and Two