An event in someone's life that must happen, in order to keep the balance of the multiverse and a development in their storyline. If you happen to change or distrupt it, the multiverse will collapse.
"Hey, i saw Jhon's girl with another man! should we tell him?"
Italian Mastermind who controlled Chicago for the majority of the late 20's and 30's. Kept his friends close and his enemies even closer. The most powerful gangster of all time.
"If you keep gaining power, they're gonna start callin' you Capone"
In terms of fandom, a canonnazi is a person who strictly adheres to canon, with no deviation whatsoever. A canon nazi is not necessarily a creator of fanfic or fanart, and can just be someone who enjoys other fanworks adhering to canon (or complains about crackfic and other non-canon compliant works).
A: Did you read the fanfic about Draco and Snape?! Omg so hot.
B: No, I don't really like non-canon pairings.
A: OMG CANON NAZI