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?
Hmmm. perhaps this book has some answers?
The Kill Box by Chris Stewart 3.65 · Rating details · 92 ratings · 13 reviews Air Force One, on a flight from the Middle East, carrying the Vice President, crashes into a North Carolina hillside. Everyone aboard has been dead for hours before the crash, struck down by a virus of totally unknown origin. The threat is immediate, blunt, and terrifying. Here is a weapon that relies on no high-tech delivery system-a weapon that is potentially more dangerous than a thousand neutron bombs.
https://www.goodreads.com/book/show/442617.The_Kill_Box
or maybe it refers to chess?
Kill Box Mate The Kill Box Mate is a checkmate pattern where a queen and rook work together to checkmate the enemy king. An important feature of this pattern is that the queen defends the rook whilst at the same time trapping the enemy king “in a box.”
https://chessfox.com/kill-box-mate/