in most forums, this is meant as a (sometimes joking) response to outrageous and or unverifiable claims by other posters. Related to pics or it didn't happen but used in cases where pictures cannot properly explain the situation or can easily be shopped.
Alice: Hey guys! I just beat through the fire and flames 100% on expert with only one hand!
Bob: pics or it didn't happen
Alice: Okay! *sends shopped picture of result screen*
Carl: That pic's shopped man, video or it didn't happen!
by Supreme_Bananas January 29, 2012