King Charles' head

Etymology
King Charles I was beheaded in 1649, but the allusion is literary, rather than historical.

Mr. Dick, in Charles Dickens' David Copperfield, lives with David's aunt, Betsy Trotwood. David establishes with Betsy Trotwood that Mr. Dick has been trying to write "a Memorial about his [i.e. Mr. Dick's] own history", but that the subject of King Charles' head keeps intruding into the text. Mr. Dick uses a discarded manuscript, with its references to King Charles' head, to create a "great kite" that he flies with David as an expression of their friendship. The narrative, written in David's voice, comments that "it was certain that the Memorial never would be finished".

Betsy Trotwood discusses Mr. Dick's affliction with the young David:


 * "Did he say anything to you about King Charles the First, child?"
 * "Yes, aunt."
 * "Ah!" said my aunt, rubbing her nose as if she were a little vexed. "That's his allegorical way of expressing it. He connects his illness with great disturbance and agitation, naturally, and that's the figure, or the simile, or whatever it's called, which he chooses to use."

The allusion was picked up by other writers and had become common by the 1890s.

Noun

 * 1) An obsession, especially one that keeps intruding irrelevantly into other matters.

Synonyms

 * idee fixe
 * white whale