A butch dyke in short-sleeves showing off very well-developed arms with well-defined muscles (but not steroid-type muscle).
Jenny: Your girlfriend has a really nice body. Her arms look really hot in short sleeves.
Carla: Oh yeah, I can barely stop myself from drooling. It's total arm porn.
Sam (Samantha): Babe, now that I've started bulking up, I'm thinking of getting a tattoo on my arm. What do you think?
Laura: Oh baby, that would totally give you arm porn.
Pictures/video of a man with well-developed arms while he is in a tank top/wife beater or shirtless. Came about in the CSI fandom after two CSI: NY episodes ("Trapped" and "Cool Hunter") which featured the character Danny Messer (portrayed by Carmine Giovinazzo) in a wife beater.
Amy: Did you see his arms? They're so...so...wow.
Susan: That's arm porn, honey. Want to watch it again?