While writing my so-called novel, I caught myself using the expression "de but en blanc", which means suddenly, without warning. According to my useful Dictionnaire d’expressions et locutions, it comes from shooting. "Blanc" represents the target, while "but" should really be spelt "butte", as it represents the little mount from which one is shooting. Hence we obtain "from the mount to the target", a straight line.
"À brûle-pourpoint" is a similar expression with a similar meaning which belongs to a similar type of vocabulary. A pourpoint was a piece of clothing for men which covered their torso from the neck to below the belt, and so "tirer à brûle-pourpoint" meant to shoot (tirer) point-blank, the gun being so close to the person that his clothes (pourpoint) would catch fire (brûler) when coming into contact with the gunpowder.
I really must stop delving into every word I write if I want to finish my novel in time.