masc. proper name, in Old Testament, Jacob's youngest son (Genesis xxxv.18), from Hebrew Binyamin, literally "son of the south," though interpreted in Genesis as "son of the right hand," from ben "son of" + yamin "right hand," also "south" (in an East-oriented culture). Compare Arabic cognate yaman "right hand, right side, south;" yamana "he was happy," literally "he turned to the right."
The right was regarded as auspicious (see left and dexterity). Also see Yemen, southpaw, and compare deasil "rightwise, turned toward the right," from Gaelic deiseil "toward the south; toward the right," from deas "right, right-hand; south." Also compare Sanskrit dakshina "right; south," and Welsh go-gledd "north," literally "left."
In reference to a favorite younger son it is from the story of Jacob's family in Genesis. With familiar forms Benjy, Benny. Slang meaning "money" (by 1999) is from the portrait of Founding Father Benjamin Franklin on U.S. $100 bill. In some old uses in herb-lore, etc., it is a folk-etymology corruption of benzoin.