Du hast aber Muskeln!
Das hast du aber verdient!
Das geht aber nicht!
"aber" basically means "but". In those cases (at least in the first one) there is no real translation, adding those filler words expresses astonishment.
You have great muscles.
You do deserve that.
No way, that's not possible.
In the third case, it can also be used to express "I've understood what you've said, BUT I have a different opinion."
I've understood your idea, but it's not possible to do it like that.
Welcome to the wonderful world of German Abtönungspartikeln. They express succinctly what would otherwise require a lot of circumstantial words, or a particular inflection of voice that doesn't translate well to the written word. Here's an approximate illustration of the effect of aber:
Du hast Muskeln = You've got muscles. (neutral)
Du hast aber Muskeln! = My, what big muscles you've got! (astonished)
You could also translate aber with really in those cases to emphasise the meaning of astonishment or, e.g. in the first sentence, admiration. The second sentence could also express a supportive attitude among friends. The last sentence also negative or complaining attitude, e.g. grannies being mad about what a young person did.
Du hast aber Muskeln.
You really have muscles.
Das hat du aber verdient.
You really deserve that.
Das geht aber nicht.
That’s really not possible. / That really doesn't work.