It is an acronym for One Night at Flumpty's and is usually used so you don't have to spell Flumpty.
Random dude 1: Have you played ONAF?
Random dude 2: You mean One Night at Flumpty's?
Random dude 1: Yeah, so have you played it?
What you exclaim when your parents tell you you can play video games for another hour because it's a Friday night.
Dad: it's okay, you can keep playing. It's a Friday night after all!
Son: Hot dog!
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