1814, "cross," from Latin crux "cross" (see cross (n.)). Figurative use for "a central difficulty," is older, from 1718; perhaps from Latin crux interpretum "a point in a text that is impossible to interpret," in which the literal sense is something like "crossroads of interpreters." Extended sense of "central point" is from 1888.
例文
1. He explained what the Crux is，or rather，what it was.
彼は南十字座とは何かを説明した、あるいはより正確には、過去の南十字座とは何かを説明した。
2.Now we come to the crux of the matter.
ここで問題の問題点についてお話しします。
3.Where does the crux lie？
結症はどこですか？
4.This is the crux of the matter.
これが問題の鍵です。
5.He said the crux of the matter was economic policy.