Sport was first used in verb to mean "pleasant passtme." However, the argument about whether or not chess is or is not a sport, comes down to its meaning as a Noun. Etymologically speaking, "sport" has meant "game involving physical exercise" since the 1500s. Queen Elizabeth knew Chess wasn't a sport, and we're still having this argument 500 years later.
The distinction between game, exercise, and sport cannot be divided by difficulty, because lifting a car is difficult, but it is neither a game nor a sport.
Luck can't divide it, because advantage is often given to one side or the other, in sports, because of the use of coin flips, seeds, and weather.
And can anyone name a sport where rules decide the outcome? I know rules decide the methodology of participation, but I can't think of a game where the rules decide the outcome. You win in hockey because you had more goals, which is fundamental, not ruling.
At the end of the day, sport is an elusive mixture between both game and exercise, that straddles the gray area on a line so thin that any tip in either direction settles the issue in its entirety.
What sport can you think of that brings so much debate to the table when you think of it existentially? At the end of the day, when you think of chess, and you have even the slightest doubt that it is not a sport, then it simply is not a sport.
You do not scrutinize real sports ("Football? Of course football's a sport"). You do not scrutinize real games ("Elements? Obviously that's not a sport"). You do not scrutinize exercise ("Man, I was pumping irons the other day, and whooboy did I win").
If chess brings scrutiny to itself, it must inevitably be lowered to its basest level; that of a game, and nothing more.
At least til they come out with Xtreme Chess.