rubi

n. [ L. rubia madder, fr. rubeus red. ] (Chem) A substance found in madder root, and probably identical with ruberythrinic acid. [ 1913 Webster ]

n. [ L. rubia madder, fr. rubeus red. ] (Chem.) One of several color-producing glycosides found in madder root. [ 1913 Webster ]

a. (Chem.) pertaining to, or derived from, rubian; specifically, designating an acid called also ruberythrinic acid. [ Obs. ] [ 1913 Webster ]

n. A ribble. [ Obs. ] Chaucer. [ 1913 Webster ]

a. [ F. ] Colored a prevailing red, bay, or black, with flecks of white or gray especially on the flanks; -- said of horses. Smart. [ 1913 Webster ]

n. [ Cf. F. rubacelle, rubicelle, fr. L. rubeus red, reddish. ] (Min.) A variety of ruby of a yellowish red color, from Brazil. [ 1913 Webster ]

prop. n. (Anc. geog.) A small river which separated Italy from Cisalpine Gaul, the province alloted to Julius Caesar. [ 1913 Webster ]

☞ By leading an army across this river, contrary to the prohibition of the civil government at Rome, Caesar precipitated the civil war which resulted in the death of Pompey and the overthrow of the senate; hence, the phrase
to pass the Rubicon or
cross the Rubicon signifies to take the decisive step by which one is committed to a hazardous enterprise from which there is no retreat. [ 1913 Webster ]

a. [ L. rubicundus, fr. rubere to be red, akin to ruber red. See Red. ] Inclining to redness; ruddy; red. “His rubicund face.” Longfellow. [ 1913 Webster ]