just fuckin'

Said when you give up halfway through a conversation or argument, or you don't know what you're gonna say. commonly followed by "Goddammit" or "ya know?"

Doing something inane and possibly stupid, for no reason whatsoever, just because the option exists.

Basically means, just do it. Just go. When someone is taking far too long and holding up your life for no known reason, you would say this to them. When sitting in the car, and waiting for another person who is not yet in the car and you just want to go.