In a group of friends, the one friend that is kept there as a backup, they do just enough to keep him/her around. Only to to stuff with when the usual crowd of friends can't. Like the spare tire on the back of your car.
Scott: Crap I've got 3 tickets and only Jake and Connor can go, Tanner said he couldn't go. I guess I'll have to see if Luke can go instead since we've got no one else.

Scott: Hey Luke want to go with us?

Luke: I guess so. Looks like I'm 5th wheeling as usual.
by Jendali December 9, 2015
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Being with a 2 sets of couples while being the only one with no person to hold or cuddle, kinda in the way but are not
Susan So you, I & Ryan, Rebecca & Bill going to the movies tonight?
Chantelle Yea but I hate being the 5th Wheel -__-
by MaR9 October 12, 2007
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This is slang for weed that is between schwag and chronic in quality. This weed is also known as 50, because it is $50 per quarter, which is where 5th Wheel derived from.
"I can't fuck with that reggie, that shit makes my head hurt. Get at me when you get some 5th Wheel."
by IIG September 18, 2008
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the 5th wheel is the spare wheel attatched to the back of a slab. the 5th completes the slab. without it you are not ridin slab. it can often be seen in a tilted or "reclined" position.
by King_Grande July 4, 2010
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The dude thats coming when they already have enough people. The dick just won't leave!!!!!!!!111ONE
First guy: Ok, I'm going with my girlfriend to see this really cool new band!
5th wheel: Awesome! Can I come?
First guy: erm...
by Lando-Commando October 10, 2007
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