Someone who complains, either in real life or on the internet, about any fight that isn't exactly and 100% "fair." This may or may not be a fight in which the fairfag is even involved. To the fairfag, every fight, no matter what the cause, is somehow a sport with corresponding rules and not a situation in which the primary goal of each party is to beat the other party up.
In some cases it is caused by general naivete and a sheltered life, while other times it is caused by disingenuous attempts to show chivalry, especially around females.
After hearing about a fight in which the winner distracted the loser and clocked him while he was looking away, Bob the Fairfag complained that it wasn't a fair fight.