That absolutely sounds like a codename from one of cstross's Laundry Files novels. (I think "boojum" was actually part of one, but I don't recall which.)
edit: found it, it was from A Colder War, which is a great novellette: https://www.infinityplus.co.uk/stories/colderwar.htm
"For the Snark _was_ a Boojum, you see."
and ended up writing the other umpteen dozen verses just so that that would make sense as a punch line.
Negative testing is trying to invalidate the sample
The hunting of the snark is written in a way that reads like "normal English" from a distance. The sentences flow fine, the words look about right if you squint. So it passes a lot of "positive tests", in that it matches our expectations for what language looks like.
You have to "negative test" the story to realize you don't know the definitions for any of the words, and that the plot is uninterpretable.
Same idea as Kahneman's system 1 that comes up with instant answers, or ChatGPT hallucinating facts by association that "look right".
It challenges the reader to try to model and define a Wodwo, but provides basically no information on what a wodwo is, aside from the fact that it is something that itself is struggles to define it's relation and connection to the world.
In my opinion, it highlights how we are all physical perception machines looking for meaning and identity, but meaning and identity can not be physically perceived.
The Snark is described in detail, with but a single additional caution that some Snarks are Boojums, with no description whatsoever of the difference. And, in the end, only a Boojum is found.
The band of snark-hunters are _also_ described in detail, almost always emphasizing the things that they cannot do or the additional risks of having them along, but they're brought along anyway.