Glamour muscles. Muscles that you work out that are only for looks - not for core strength.
From the show "it's always Sunny in Philadelphia" this is what Dennis called Mac's muscles - GM - Glamour Muscles - god dammit!
Dennis: Well, how the hell are you planning on getting in there, Mac?
Mac: Uh... you put me on the spot. First thing that comes to my head, scaling the façade. All right, I know I soured on that earlier, but I believe that is my mind telling me that I have it in me.
Dennis: Okay, I hate to tell you this, bro, but you do not have the core strength to scale the façade of Citizens Bank. You just don't.
Mac: What? I work out all the time!
Dennis: Yeah, but you only work out your glamour muscles, and you know it.
Mac: I work out my core.
Dennis: No, you do not work your core. You're totally arm-heavy. You're all bis and tris and everything else is just fat and-and ribs.
Mac: Bro, I can do way more push-ups than you, and that's like 16 different muscle groups, I think.
Dennis: That is beyond retarded, what you’re saying right now. edit