Brookline third-grader Beau Keough considers himself more than a dedicated lacrosse player. At 9 years old, he is already a lax bro, one of a growing group of young lacrosse enthusiasts who have cultivated a laid-back lifestyle to pair with the high-energy field sport.

'You have to have a lot of the stuff, wear it a lot of the time,'' said Keough, who plays on the town team. 'Lacrosse is cool.''

Older lax bros ' short for 'lacrosse brothers' ' tend to offer similarly vague definitions of the phenomenon, but it is easy to spot them. Lax bros display a certain understated confidence that critics call arrogance. They wear their hair long, a look known as 'lettuce.' They dress in colorful board shorts, flat-brim hats, and bright half-calf socks. They carry lacrosse sticks, or 'spoons,' on and off the field.

Everything good is 'chill,' especially chillin', a favored activity that often includes listening to the music of O.A.R., Dispatch, and Dave Matthews.

The attitude and style are proliferating on school campuses from junior high schools to universities as the game has hooked tens of thousands of young athletes nationwide in recent years.

First played by Native Americans hundreds of years ago, lacrosse is the fastest growing team sport in the United States, according to the Sporting Goods Manufacturers Association, based in Maryland. The game is luring players from the rosters of Little League and other youth baseball teams and attracting businesses eager to profit from specialized lacrosse gear, camps, clubs, and pricey apparel.

Supporters of the subculture say it provides young athletes with a sense of identity and is helping to popularize a sport that is uniquely American.

'You check the way the kids carry themselves,'' said Tommy Lee, a father of two lax bros who coaches a Brookline Youth Lacrosse team of fifth- and sixth-grade boys. 'It has become a really cool thing. It's like walking around with a little swagger.'