E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Dignus Vindice Nodus (Latin).
A knot or difficulty worthy of such hands to untie. Literally, a knotty point worthy to be made a civil action. The person who brought a civil action was called in Roman law a vindex, and the action was called a vindictio. If the rightful possessor was a matter of dispute, the question became a lis vindicirum, and was referred to the prætor to determine. A knotty point referred to the prætor was a dignus vindice nodus.