Registration plate. Number plate. Motor vehicle registration license number plate.
Relatives, kin, the family.
Originally a ring-in was a good horse covertly brought into the parade ring to substitute for a horse with a bad name so it could win with high odds. Now a ring-in is any last-minute replacement.
Things have been displaced in that person's head.
Never, ever, say you want to root for someone's team. You want to barrack for them, not get busy in the barracks.
An individual who is eternally alert for intimate encounters.