To raise the index finger to in order to show that you are angry, pissed, annoyed or otherwise cross with someone. It is also called "flipping someone off".
Person 1: The psychiatrist told me that I had Oppositional Defiance Disorder because I did not want to take the over-prescribed drug for being "over-active".
Person 2: Dude, are you going to let that guy give you the middle finger?! (NOTE: figurative use)
The thumb and middle finger pose can be used in things such as photos and videos, and may also be used to identify other people who, much like yourself, have a small penis.
(n.) when you flash someone the reverse middle finger, it is to say “no fags” when someone is either being too gay or acting in a gay manner and you want none of it.
Tommy: Tim was acting hella gay grabbing all the boys cocks at the party so we all hit him with the reverse middle finger to let him no he was being a fag