Nurse Hopkins mixes her metaphors

Whenever I think I have caught Agatha Christie in the act of committing a mixed metaphor, I realise that it's not the author but the character who is in error. Nurse Hopkins is just the sort of person who would remark (to Hercule Poirot):

"And I'll thank you, Mr. Poirot, to keep any nasty insinuations to yourself ! There's not a thing about Mary Gerrard's death that I haven't been open and above-board as daylight about".*

Poirot, though at times he does profess to be baffled by English idiom, seems on this occasion to have understood and accepted Nurse Hopkins's protest unperturbed. He might have wondered, though, how far above board daylight is. What is it like below board, then ? Darkish, presumably.

The 'board' metaphor only makes sense once you realise that it refers to the table at which you play cards - and below which you might try to fiddle them. 'Open as daylight' is a rather different metaphor, but could be used quite correctly in parallel with 'above board': it is the 'comparative' preposition 'as' that so unsatisfactorily mixes the two metaphors.

* Sad Cypress, chapter 13

Nurse Hopkins mixes her metaphors