Mounterbarnks are fake so put it in I am a mountebank that does not help so think your self!
"Unction" is an oil or ointment; a "mountebank" is a charlatan or quack who sold patent medicines, so the unction of a mountebank is some concoction sold by this quack. In the context of Hamlet...
Here's one: Mountebank is a word. Hoped this helped!
Chapter 8: The Elf-Child and the Minister