For an amusing example of this idiom, go to YouTube and search for "ella fitzgerald undecided". This song is all about being undecided:
0:20 "First you say you do, and then you don't;
And then you say you will, and then you won't;
You're undecided now--so what are you going to do? ...
1:00 I've been sittin' on the fence
And it doesn't make much sense
'Cause you keep me in suspense
And you know it!
Then you promise to return,
When you don't, I really burn,
And I guess I'll never learn
And I show it!