In the era of smartphones and showing-off on social media, to believe anything far-fetched, photographic proof is expected. The above phrase simply says that if one doesn't have proof, the claimed thing doesn't exist.
Also see : "Pics or it didn't happen"
A : Hey Bro, I just got a Harry Potter 1st edition book, autographed by J K Rowling herself !!
B : Pics or it doesn't exist
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.
I had a trucker's tan after driving for 10 straight hours on a sunny day.