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.
crux (krŭks, kruks)
n. pl. crux·es or cru·ces (krōō'sēz)
A cross or a crosslike structure.