third wheel

Short definition from Wiktionary

Noun

1. 1
   An individual or object that serves no useful purpose.
2. 2
   A third party, usually unwanted, accompanying two people on a date.
3. 3
   Used other than figuratively or idiomatically: see third, wheel.

Verb

1. 1
   Alternative form of third-wheel.
2. 2
   To act as a third wheel; to accompany two people, unwanted, on a date.