When playing the card game euchre and one team has obtained nine tricks or points, one teammate does the following:

1. announce to opponents and partner "Dude! we're in the barn!"
2. balance the scorecards on ears (one on each)
3. link fingers together and outstretch hands with thumbs facing down, thus allowing their teammate across to "milk" his/her thumbs
4. both teammates moo similtaneously

a person who tries too hard to fit in, poses, is easily and obliviously used and is in general just a stupid fuck. so much to the point that this person isn't just a tool, they're equivalent to an entire sack of tool. therefore, they're a toolsack.