When you are working in a team, you often make branches for features/bugs. After the branch gets merged into master, you end up with an old reference in your own git history.
If you are like me, that although knowing git in the command line, use the Github Desktop app, you end up with an infinite list of old branches.
Heres the one single command solution:
$ git branch -vv | grep 'origin/.*: gone]' | awk '{print $1}' | xargs git branch -d
Reference: http://erikaybar.name/git-deleting-old-local-branches/