The origin of the idiom 'on the fence' has its roots in Middle English, when the word 'fens', short for 'defens' was originated. Later, an “e” was added to the term to form the word defense as it is used today.
What does it mean to say on the fence?
If you're on the fence, you can't decide something. You're torn between two options. ... Being on the fence means you really just can't decide. It's as if you're sitting on the fence because you can't decide which yard to play in.