De but en blanc

Following the examination of Poirot's Christmas at reference 1, today we are at the secret of Chimneys, with Chimneys being the name of a house in the country, a house grand enough to run to a butler amongst other servants. In the course of which I come across the expression 'attaquer de but en blanc' for 'to set out in cold blood to fascinate'. This being what a lady is being asked to do to a gentleman on page 233 of the Heron edition.

But the exercise was to find out what the French expression meant without recourse to the English. The general idea was reasonably clear from the context, but I wanted to do a bit better than the general idea. So out with the little Littré, always good for a treasure hunt.

After a few false starts I find 'tirer de but en blanc' under blanc (noun), a word with an interesting range of meanings spanning our white and blank. A blanc is, for example, the name for the blank, white painted shield of the knight in armour too young to have earned the right to carry any heraldry. It is also the name for the white spot or patch on a target used for practice with a bow, a gun or more particularly an artillery piece. But I can't understand the explanation of the tirer business; something about the curved trajectories of cannon balls when the cannon is pointing directly at the target.

Try 'tirer' but that is hopeless. The word attracts two two column pages of close packed meanings, and I was not going to wade through that lot.

Next stop 'but', where it seems fairly clear that the word means objective or target. Which fits with the English butts. But then getting warm, we are told that the tirer phrase is a technical term used by 17th century French artillery officers. More talk of curved trajectories. But also a suggestion of firing at extreme range (toute portée) - when the cannon would certainly not be pointing directly at the target. By extension, firing wildly. And by further extension, doing something without any preparation, brusquely or without thought. Thus getting rather closer to Agatha's meaning.

Littré more or less exhausted at this point, so off to the internet. The phrase is clearly of great interest with google coming up with all kinds of places offering to explain it. Reference 2 looks hopeful so I try that, a site presumably dedicated to artillery buffs.

But which contains the confusing information that the but or butte is the little mound on which one put one's cannon, not the target at all. So doing something from the start to the end, which is not unreasonable but which is confusing.

Another site talks of firing directly at the target. No mucking about with range or anything like that. So what is the talk of curved trajectories?

Another talks of firing at mid range, at what it calls portée moyenne. With the same site talking of the English equivalent being point blank. The first according badly with the toute portée above, but with the last according rather better with brusquely etc.

Perhaps the right answer is that there is no right answer, no simple explanation of the phrase, Just various possibilities, various combinations, a situation which I now recall from explanations of some of our English idioms.

Reference 1: http://psmv2.blogspot.co.uk/2014/02/le-noel-dhercule-poirot.html.

Reference 2: http://basart.artillerie.asso.fr/article.php3?id_article=1168.