When there is a couple together, and one extra person there without a date, so they are 'third-wheeling' the couple.
Mary: You and Your boyfriend (Jack) go to the movies together.
Wendy: It will be boring without you though, can't you come?
Mary: Only if we invite another person so Im not a third wheeler
Wendy: It will be boring without you though, can't you come?
Mary: Only if we invite another person so Im not a third wheeler
by RRRRRRAAAAACCCCCHHHHH September 30, 2013
A third wheeler is somebody that tags along with camila cabello and shawn mendes when they're out. They're so sickening cute together, that you just look at them, feeling lonely, being a third wheeler.
by memeing July 30, 2019
by BobbyExDeeDee August 08, 2015