To do something that isn't really necessary.
A child in a furniture shop touching everything that is in sight. Mum might say "why are you being farce for?" Meaning that the child is touching things which he doesn't need to be touching.
You are in town and there is a random guitar on the floor so you decide to pick it up and try and play it. Friend says "you're so farce". Meaning that you're extra because you didn't have to pick it up when there was no need.