Acronym of 'you're a Nazi'. Similar to YLAN ('you're like a Nazi') or past tense YANed.
Martin Von Ehrlichman hypothesised in 2004 the longer any online argument lasts, the higher the chances of one party accusing the other of either being a Nazi, or saying their actions are attributed to the Nazi party.
After YAN, or YLAN has been said, the person who resorted to the phrase is said to have lost any argument, as attributing anyone to the Nazi party is the easiest, and therefore most imbecilic kind of insult.
Person 1 - "I can't believe you said that, you're like a Nazi or something!"