bras d'honneur
"Arm of Honor" in French, a rude gesture usually consisting of one arm, bent at ninety degrees and turned upward, while the opposite hand sits in the crook of the elbow or grips the bicep. What someone chooses to do with the hand of the upturned arm may differ.
Commonly seen around the world as a full-body "fuck you", equivalent in meaning to the, and often used together with, the middle finger.
In Japanese culture, however, it's perceived as a form of muscle flexing, indicating confidence and determination. See Danbooru topic #9716.