Did Q ever refer to it as such? It's been part of the general lexicon for so long that I can't even remember it's origin.
When was it first used? Why is it referred to as a killbox?
In military language a killbox is a 3D target area defined to facilitate the integration of joint weapons fire.
Was there anything that Q said that marked these bracket usages as such? Or was that an anon idea that took off?
The first time I can recall seeing the "[insert name here]" bracket symbols being referred to as a "kill box" is from old video games. When an enemy player was targeted on a video screen, their name would get brackets placed around the player name to give a visual indication that the player was targeted for attacks. I always thought the term "kill box" was a slang reference based on that. But it may have originated from some other lexicon too.