Usually written '+1', used in a forum post to indicate that the post serves no purpose other than to increase the poster's postcount. Usually used after an empty post along the lines of 'I agree'. Often used in a sarcastic manner to indicate the pointlessness of the entire thread.
(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!"