Definitions
1.NounA move in square dancing in which two dancers face each other, then step forward and left until they have right shoulders adjacent, then move to a position where they are back to back, then move to have left shoulders adjacent, then return to facing each other.
2.NounA sofa, open carriage, etc. constructed so that the occupants sit back to back.
3.AdverbBack to back."to sit dos a dos on a camel; to dance dos a dos, so that two dancers move forward and pass back to back"