What does 上不上下不下 mean as an idiom? (Not the riddle)
Here is the sentence where the phrase came from. 可惜，她越是这样，楚野就越是觉得被一根刺卡在喉咙处，上不上下不下，非常的不爽。
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「上不上下不下」has three meanings; the first one listed applies to your example sentence.
Alas, the more she (她) acts this way, the more upset 楚野 feels, as if there is a thorn stuck in her (楚野's) throat, 「上不上下不下」.
Two options for translating「上不上下不下」:
Referring to the thorn stuck in the throat: unable to be spat out or swallowed.
As an emphasis of the feeling of being upset (「非常的不爽」): making her (楚野) extremely uneasy.
Neither translation alters the meaning.
上不上下不下 is not a standard idiom。 It derives from 不上不下. The new form (上不上下不下）is easier for modern people to understand. People often misunderstood the meaning of 不上不下， so the writer made it easier for people to understand. The same thing is happening to English. Uneducated people like saying “anyways “, but it should be “anyway”. There is no “s”. Now , more and more people say “anyways “, so some writers start to use “anyways”.
It's a figure of speech here. It denotes the sense of uncertain or undecided. Usually, what make people uneasiest/most uncomfortable is that something important/you care most is undecided or uncertain.
In this context, what she had done put 楚野 into a situation where he felt most uncomfortable and didn't know what to do, just like a thorn stuck in his throat which could neither came out of his mouth nor be swallowed into his stomach.
P.S. 上不上下不下 literally can be taken as 说上不上, 说下不下: If you say it's 'up', it's not really 'up'. If you say it's 'down', it's not really 'down'. (And it's right there)
上不上, 下不下 is something between a detailed sentence and an idiomatic phrase
Can't go up, can't go down (struck in the middle)
要上不能上, 要下不能下 --> 上不上, 下不下 --> 不上不下
Usually, we would reduce 要上不能上, 要下不能下 directly to an idiomatic phrase "不上不下"
Example of similarly structured phrase:
説咸不咸，説淡不淡 --> 咸不咸, 淡不淡 --> 不咸不淡