'Couch' in the Bible

Now [we come to] the sons of Reuben the firstborn of Israel—for Reuben was the eldest, but because he defiled his father’s bed [with Bilhah his father’s concubine], his birthright was given to [Manasseh and Ephraim] the sons of Joseph [the favorite] son of Israel, so that he is not enrolled in the genealogy according to the birthright.