a fifth wheel

a fifth/third wheel

  (American)
someone who is in a situation where they are not really needed or are ignored by other people I don't have a role in the office any more - I feel like a fifth wheel.
See also: fifth, wheel
References in periodicals archive ?
A heavy load pulled across a smooth surface places less stress on a fifth wheel than a light load pulled in severe, off-highway conditions.
All glass in a fifth wheel travel trailer, including windows, interior partitions, skylights and doors must be safety glass.