(noun) - the person who is invited to join an existing coupled unit in sexual activity resulting in a three way or menage a trois. Depending on the genders and desires of the couple, the "plus one" may even become a lucky pierre.
"'allo, are you two a couple? Fancy a plus one?"
"Nice accent. Where are you from?"
"Australia. And I'm only 16!"
"But I'd make a great Lucky Pierre!"