Nickname for has-been or never-will-be female models who grow a large social media audience by posting frequent bikini, side-boob, and fitness pictures. Because they can't land legitimate modeling or acting gigs, they are relegated to promoting protein shakes and supplements on Instagram, Snapchat, and Facebook.
Protein slingers can be male, but they are very rare.
Ed: "Hey, have you ever hear of this @NatalieBurk89 chick on Instagram?"
Amit: "Nah, man ... I can't tell these protein slingers apart any more"
Ed: "True"