What is the X in X-mas?
masc. proper name, Old Testament eldest son of Jacob and name of the tribe descended from him, from Greek Rouben, from Hebrew Reubhen, probably literally "Behold a son," from reu, imperative of ra'ah "he saw" + ben "a son." As a typical name of a farmer, rustic, or country bumpkin, from 1804. The reuben sandwich (1956) is "Not obviously connected" with the sense in rube [OED], possibly from Reuben's restaurant, a popular spot in New York's Lower East Side.
behold a son!, the eldest son of Jacob and Leah (Gen. 29:32). His sinful conduct, referred to in Gen. 35:22, brought down upon him his dying father's malediction (48:4). He showed kindness to Joseph, and was the means of saving his life when his other brothers would have put him to death (37:21,22). It was he also who pledged his life and the life of his sons when Jacob was unwilling to let Benjamin go down into Egypt. After Jacob and his family went down into Egypt (46:8) no further mention is made of Reuben beyond what is recorded in ch. 49:3,4.
one of the 12 tribes of Israel that in biblical times comprised the people of Israel who later became the Jewish people. The tribe was named after the oldest of Jacob's sons born of Leah, his first wife.