1. a member of a team that has no real purpose but gets the same credit as the rest just by being a part of said team.
2. a member of a team that isn't needed and usually gets in the way, without him/her the functionality of said team would remain the same.
Comparative to an actual buttstock where the rifle would work just as well without it.
1. "look at Andy, he just slows us down and still gets the same pay as we do. What a buttstock"