Someone who consistently uses grammar in all aspects of their posts or comments on Internet websites or games, particularly in areas where the main focus with communication is on speed and messages that are 'short and sweet'. These people often present themselves as elitist and often criticise and attack others who do not use the same amount of correct spelling and punctuation as they do. Grammarfags often tend to dismiss comments by others as incomprehensible if the comment does not contain full correct spelling and punctuation.
Guy 1: man that guy is such a grammarfag, who spends more time typing than shooting on an first-person shooter??
Guy 2: Whoa. What was that? I only speak English.
on an online forum
Guy 1: Okay guys, post some positive feedback for me!
Guy 2: gr8 work but wut i think u shud do is...
Guy 1: I'm going to ignore that comment because it I can't read it. Please have some human decency and use grammar for once.