Crux \Crux\ (kr[u^]ks), n.; pl. E. Cruxes (-[e^]z), L. Cruces (kr[udd]"s[=e]z). [L., cross, torture, trouble.]

Anything that is very puzzling or difficult to explain. --Dr. Sheridan.

The perpetual crux of New Testament chronologists. --Strauss.

Source: Webster's Revised Unabridged Dictionary (1913)