數學上的第 n 個
這是字面本身，其他用法都從這裡借走感覺。
adjective · B2
In maths and computing, nth means 'at position n': a place in a list or a sequence that you have not tied to one number, so that a single rule can cover every place at once.
第 n 個的
核心畫面
One way to hold this word: n is a place-holder for any number at all, and -th turns it into a position, the way four becomes fourth. So nth points at a place in a line without saying which place. From that one idea, two everyday uses grow: a position nobody has bothered to count to, and a height nobody can reach the top of.
可以想成：n 是一個代替任何數字的空位，-th 把它變成順序，就像 four 變成 fourth。所以 nth 指著…更多收起
可以想成：n 是一個代替任何數字的空位，-th 把它變成順序，就像 four 變成 fourth。所以 nth 指著一排東西裡的某個位置，但不說是哪一個。日常英文用的兩個意思，都是從這個「沒有指定是第幾個」長出來的：一個是「多到懶得再數」的那一次，一個是「高到沒有盡頭」的那個程度。From the word’s history
From n plus -th, the ending that makes ordinary numbers into positions (four to fourth), so nth is 'position n'.
由 n 加上表示順序的 -th 組成（就像 four 變 fourth），所以 nth 的字面就是「第 n 個」。
Wiktionary entries for nth, supplied in this packet (kaikki:3f6f69a190036ae4:868285911 and :868301832): 'From n + -th (ordinal suffix).'
Two branches
借 n 來講「數不清」和「沒有上限」
n 本來就是「沒有指定是幾」。日常英文把這一點放大，用來講「多到不想數」和「高到沒有頂」。
adjective · C1
Used in front of a noun to say that this is one more in a long line of the same thing: you have lost count of how many times it has happened, and you are usually tired of it.
不知第幾次的