Webster’s Revised Unabridged Dictionary · 1913
Crux
Crux (krŭks), n.; pl. E. Cruxes (-ĕz), L. Cruces (krṳ′sēz). [L., cross, torture, trouble.] Anything that is very puzzling or difficult to explain. Dr. Sheridan.
The perpetual crux of New Testament chronologists. Strauss.