A person who loves sports as an entertainment outlet and is highly interested in one or more sport or sports team.
At home, there is the possibility the sports fanatic will stare into a trance in front of the tv in which case you can wave your hand in front of their face just to remind them they made a choice to be hypnotized and you are not responsibile.
At a game, there is the possibility the sports fanatic will shout various words out of loyalty about his team or the opposing team or about his opposing fans that will be rude, crude or just downright funny.
Stay away from people who like to attract security guards at games for various reasons.
There is also danger the entertainment derivatives of sports can take up so much time in one's life that reality becomes the outfield and fantasy becomes the infield.
Her boyfriend (the sports fanatic) owns and wears many shirts/jerseys/hats with team logos, has sports memorabilia on his walls, goes to many games, and becomes depressed when his favorite team loses. On an otherwise leisurely Sunday morning, he gets up early to chat with his friends (who help keep the obsession going) about which picks are good for online sports betting. Maybe if he just played the sport with his friends at the local park, he'd have less energy to be a sports fanatic/overly concerned fan and sweat out the obsession. She tries to remind him that professional sports players are swimming in astronomical amounts of money that shouldn't be encouraged by the plain folk. She thinks that the overindulgence of extra money, time and attention this fanatical fan gives to sports, could be better spent on his own life in which more excitement and variation of activities and hobbies might be needed and prove more rewarding.