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.
1. He explained what the Crux is, or rather, what it was.
2. Now we come to the crux of the matter .
3. Where does the crux lie?
4. This is the crux of the matter.
5. He said the crux of the matter was economic policy.