Matthew McConaughey's Weight Hits Rock Bottom

Matt McConaughey -- or what's left of him -- arrived at LAX this weekend looking skinnier than ever ... just as he planned.

As we previously reported, the usually buff and shirtless 42-year-old is wasting away to play Ron Woodruff in the movie "The Dallas Buyers Club" -- the true story of a man who contracted HIV through drug use in the '80s and has to smuggle life-saving medications from Mexico.

Matthew is reportedly slated to begin production on the movie in New Orleans today ... and he's reportedly the lightest he's ever been ... down more than 30 pounds from his usual weight.

According to People.com, the actor plans to keep his current weight for another month ... and then the weight gain begins.