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5.
(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!"
"See ya'!"
"But I'd make a great Lucky Pierre!"
by whirled-traveler January 15, 2009
 
1.
your guest when you are invited to an event
"billy's gonna be my plus-one at jane's party"
by c-line April 25, 2004
 
2.
Accompanied by a guest.
"Can you put me on the list for the 'Matrix' screening? I'll be plus one."
by Matthew Michaud January 19, 2004
 
3.
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.
I agree.

+1
by Emperor Spleen February 04, 2004
 
4.
At public parties and nightclubs, the guest of a person who is on the guest list, and who normally pays a discounted entry fee. Also, the state of having such a person.
"You are on g list plus one. Do not mention this to anyone."

"You can be my plus one."
by Wrench December 16, 2007
 
6.
To press the "+1" button in Google+ if you like someone's stream post. Roughly equivalent to the Facebook "like" function.

Pronunciation guide: plʌswʌn or "plus" + "one"
I just plusoned you ;-). I hope you plusone me back.
by angrymonkeyman June 30, 2011
 
7.
n., a person who sleeps with (and therefore enjoys many of the social priveledges administered to) an entertainer, especially a Disc Jockey
Where'd he find THAT hott plus one!?!?!
by mwikkid April 03, 2005