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The easiest way to tell if a picture is a selfie, especially when the selfie taker denies it.

Selfie shoulder is where one shoulder is way too far forward in that picture because the selfie taker is stretching that arm to aim the camera back at themselves.
"That's not a selfie! My friend Reese took it!"
"Girl, that big ol' selfie shoulder says otherwise."
by Dolores Del Rio January 31, 2014