E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A knotty point, a difficulty. Instantia crucis means a crucial test, or the point where two similar diseases crossed and showed a special feature. It does not refer to the cross, an instrument of punishment; but to the crossing of two lines, called also a node or knot; hence a trouble or difficulty. Quæ te mala crux agitat? (Plautus); What evil cross distresses you?i.e. what difficulty, what trouble are you under?