The rather smelly state of your fingers after taking the quite disgusting step to travel on public transport. If you have the inclination to touch the seats on the bus/train/cab the result is "bus finger".
Can also be used as a verb post-travelling on the bus. Eg. "Hey, you just bus fingered my food!" Can also be used as an adjective "Look at that bus fingered coffee mug!" Some people don't mind the state of their own bus fingers but hate other peoples.