A straight line, with nothing on it. You are so close to the white spot on the target that you do not have to aim above it and wait for the bullet to drop. You point at the mark and it is hit. That flat, unadjusted line is the whole word. Move it from a gun to a mouth and you have the other half of the entry: a question or a refusal that goes straight to the person, with no cushion on the way.
From the word’s history
Wiktionary derives the word from French point blanc, white point — the white spot aimed at on a target — with an alternative account of firing horizontally into empty space. Either way, the root names the thing you aim straight at, which is why the modern word is about a line that needs no adjustment.
來源記載這個字來自法文 point blanc（白點），也就是靶上那個要瞄的白點；另一說是指水平射入空處。不論哪一說，字根講的都是「直直瞄準的那一點」，所以今天這個字的核心才會是一條不必修正的直線。
Wiktionary entry for point-blank (kaikki:3f6f69a190036ae4:1072838397:11529): from French point blanc (white point), originally the white spot aimed at on a target.
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