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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.
constellation lying at about 12 hours 30 minutes right ascension (the coordinate on the celestial sphere analogous to longitude on the Earth) and 60 south declination (angular distance south of the celestial equator), now visible only from south of about 30 north latitude (i.e., the latitude of North Africa and Florida). It appears on the flags of Australia, New Zealand, and Samoa (formerly Western Samoa)