It's a common name for the plant Sagittaria. I'm pretty sure this is mentioned in the book (though it's been a few years since I read HG). I'm not sure what Katniss itself mean but the Etymology Dictionary says Sagittarius means "archer," "pertaining to arrows," from sagitta "arrow," which probably is from a pre-Latin Mediterranean language. http://www.etymonline.com/index.php?allowed_in_frame=0&search=sagittarius&searchmode=noneWhere did you read that it was used before Hunger Games?
Where did you read that it was used before Hunger Games?