Emily Dickinson Lexicon

RU'BI-CON, n.

A small river which separated Italy from Cisalpine Gaul, the province allotted to Cesar. When Cesar crossed that stream he invaded Italy, with the intention of reducing it to his power. Hence the phrase, to pass the Rubicon, signifies to take a desperate step in an enterprise, or to adopt a measure from which one can not recede, or from which he is determined not to recede.