There's two different meanings. (See example below.)
Girl: "I just wanna be...friends with benefits," usually means: "Yeah, you're cool but...I don't really wanna be seen with you. Sorry." Guy: "I just wanna be...friends with benefits," usually means: "Yeah, I just wanna get in your pants without having to do anything for you."
By taaash
When a boy and girl who are good friends use each other for kissing, hugging, holding hands, etc. These acts, (commonly done privately/under the radar) usually work until one person falls for the other. This normally leads to one heartbroken person and two friendship less people.
Mark- wanna me friends with benefits? Katie- what are the benefits? Mark- well we could kiss Katie- maybe, what if I get hurt? Mark- don't worry you won't *2 weeks later* Katie-you asshole, I hate you. Mark- what did I do? Katie- just stop hurting me we're done *no more friendship*
By Miss. Fluffles
Friends is perhaps a little too generous here too but appropriate if you and a person who already had an established friendship drink a little too much spiced rum one evening at a party after everyone else has gone to bed and decide that you suddenly find each other attractive. It may be possible for you both continue to stay friends and occasionally root each other but eventually your friendship will be ruined for an approximate period of three years. After that time you may be capable of laughing about it without feeling awkward. Meeting someone for the first time, having sex and then trying to develop a friends with benefits type relationship can only work for a short period of time because you did not become friends first. Calling yourselves friends is just a simple excuse to continue to root without having to deal with the difficult conversations about who leaves the toilet seat up.
You know, I never thought you were hot before but wow, you truly are beautiful. maybe we should become friends with benefits.
By Relationship Guru
it consists two friends who agree to have a physical relation without officially date.
Amy and Alex aren't dating, but they are having sex togeather. That means they are friends with benefits.
By trula
Usually, a couple of friends who desire a physical relationship with eachother, but not involving any emotional ties with one another.
The guy gets some action, and the girl gets used and left out to dry.... "friends with benefits"
By Kellan