in the figurative phrase cross (or pass) the Rubicon "take a decisive step," 1620s, a reference to a small stream to the Adriatic on the coast of northern Italy which in ancient times formed part of the southern boundary of Cisalpine Gaul. It was crossed by Caesar, Jan. 10, 49 B.C.E., when he left his province to attack Pompey. The name is from Latin rubicundus "ruddy," in reference to the color of the soil on its banks.
Strictly one of the names of Memphis, it was taken by the Greeks as the name of the whole country. The Egyptian name, Kemet, means "black country," possibly in reference to the rich delta soil. The Arabic is Misr, which is derived from Mizraim, the name of a son of Biblical Ham.
The name was also used to signify the evil inherent in human nature (as a consequence of Adam's fall), and other qualities (nakedness, gardening) associated with the biblical Adam. Adam's ale "water" is from 1640s. To not know (someone) from Adam "not know him at all" is first recorded 1784. The pet form of the name in Middle English was Addy, hence Addison; other old pet forms (Adkin, Adcock) also survive in surnames.