A legit chiller is a generally dumb high school-aged male who parties too much but hence are insanely good at beer pong. They have huge egos but people who aren't as "chill" as they are tend to hate them anyway. They all tend to come from upscale, boring families and spend all their dirty money on liquor, cars, good hash, and clothes. Legit chillers differ from their neighboring frat-dick subculture, bros, in that they play lacrosse rather than frisbee, tend to listen to rap and house music rather than alterna-crap like Incubus, and their attitude towards everything is always to fuel their ego rather than be as "chill" as possible. While this does contradict the "chillers" aspect of their name, their definition of what is "chill" is defined by them and hence, supposedly legitimate (legit), which lets them think they are better than everybody who cannot chug 60 oz of beer in two minutes and don't drive a BMW.
Legit Chiller 1: "Yo man this party is going to be rad... all the legit chillers are gonna be there, and I heard Langston has 8 oz of killer hash he got when his father took him to Columbia..."
Legit Chiller 2: "That's chill, bro. Make sure no stupid freshman come, dude, they're not legit."
Freshman: "Fuck that it's gonna suck anyway. I'm going to go play game cube or something...."