I went to grade school with someone who was exactly this trope. All he would talk about every day was his kung fu (wushu) prowess, and he'd throw about the names of fighting styles, weapons and other terminology for show. For Chinese class in school, we kept a "weekly journal", and sometimes the teacher would allow us to write entries on any topic of our choice. The teacher complained that when it came to the "arrogant kung fu guy", almost all this type of journal entry were about kung fu, and he would very often "practise" new moves that he had learnt - on me. My testicles ached for a week. The funny bit is, he was the guy who would get injured the most often: he fractured his leg twice in a year, and one of the times I was made to help him out around the school and stay with him in the classroom as everyone else went down to the cafeteria for recess.

Same troper as above: One of my good friends is an American expatriate who moved here for a bit and was homeschooled, so a large part of his education involved taking classes for different skills. One of the classes he would take was Taekwondo, and he would brag about all the little kids he had punched out - all the time. I'm not sure if there's something about me that attracts this very specific type of friend.

about 13 years ago, there was one of these types, an assisstant instructor at this troper's dojo. He wore a blood red uniform, and did nothing except brag about all of the people he'd beaten up in matches; as an instructor, he was both condescending and incompetent. When ever he was matched up with someone smaller or weaker than him, he would show off by pushing them around, and he would be an especial bully whenever he'd bring his girlfriend to watch him. One night, he was sparring with the guy who now runs the dojo. Since his girlfriend was there, and his opponent was about a foot shorter than he was, he decided to show off. His opponent fired a jump spin kick to the back of his head, and the AKFG smashed face-first to the floor. He never was seen again.

I know a guy in Chicago who believes, with all his heart, all the myths you hear about Shaolin Monks. He believes that you can poke your finger at a hanging weight, and learn to move it without touching it. Then, stand ten paces away from a candle, and put out the fire by poking your finger at it. THEN do the same thing with a pane of glass in the way. The end result? Finger poke of death. It kills you in a week unless you eat a "special herb". He thinks all these things are true.

This troper has come up against a couple of these guys during his last five years as an amateur mixed martial artist. They deflate quite quickly....

This troper knew one in high school, he came out of Middle School having been bullied for being an Otaku, and the metamorphosis was downright scary. He went from a good natured kid to always quiet, gave off an air like, I can't explain it. But you just feel in danger around him. He became even worse when he started explicitly copying Rob Lucci. He once fought a guy carrying two knives with one hand held behind his back...and overwhelmingly won.