I like to flip off my coworkers in my pocket while at work
Why would you secretly give your co-workers the finger?
They are mean people who make my life hell at work with never ending personal attacks.
The people you work with may not treat you as you would expect in a work environment, but flipping them off secretly while smiling to their face is a demeaning act of retribution. Revenge will only get you more more of the same ugly behaviour from your co-workers.
I was only showing them my opinion
It still seems childish to behave this way. If you have a problem why don't you tell them to their face in a respectful tone. If you confront them they may be willing to stop.