Top Definition
Phrase used in pretty much every conceivable scenario when person#1 makes a claim that requires proof to be believed, and person#2 demands that the proof be given, or person#1 shuts the fuck up.
Pietro, Ultra, Earl Grey: Hey! That's not fair ACM! "proof or stfu" is our phrase that we invented!
The ACM: proof or stfu
by TheACM December 16, 2006

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