From what I've read on these forums a "nit" is someone that won't end his days in a shitty motel without a dime to his name. Seriously, a lot of behavior that people on here call "nitty" is just common sense. For instance if you are a C-player, refusing a big money game against an A+ player is usually the smart thing to do. He probably won't give you the kind of handicap you'd need to win, which would sound ridiculous. People are very bad at calculating the odds of winning. For someone who's able to run 3-5 balls (for instance) they are extremely low against a high level player, and in fact that kind of player almost always win these kinds of bets. The real weight that needs to be given sounds exceessive and anyone who demands them will be called a nit.
I think the word nit is more like a strategy used by gamblers to bully players into playing, more than an apt description of a persons character in about 99% of the cases. There is a clear tendency of the term to be used on weaker players, when in fact it would be more aptly applied to the high level ones. They are the ones with the experience to calculate the odds, yet they rarely give anybody a fair game. A nit, to them, is someone who refuses to pay for the privilege of playing them, by agreeing to play without adequate weight. They will toss the term around liberally, yet when you look at their winning percentages and the ratio of games won and lost, it gets pretty clear who the real nits are... I lost plenty of such games in the beginning when I was learning pool, but unlike a lot of people I knew what was actually taking place, and never got tricked into doubling the bet etc.