I've been watching Bron/Broen, and the Danish characters this season all seem to love saying, "Hold kæft, mand!". In some cases, like with Lukas Stenstrup, it's obvious that "shut up" is the translation. In others, however (e.g. when Thure Lindhardt whispers it upon seeing a crime scene for the first time), it's not so obvious.
I could see it being "come on, man" or "you've got to be kidding me", but I'm really just guessing at the sentiment. Native speakers and folks living in Denmark: what do you consider it to be? And when the 'mand' follows, would you include it in a translation?
In the situation where he see the crime for the first time, it is more like a statement of surprise: "Wow, that is wild" or "wow, that was crazy/extreme", so your suggestion "you've got to be kidding me" is quite allright. :-)
Danish people also sometimes use "Hold kæft" to intensify the expression of a feeling - if you are very, very tired you could of course say "jeg er meget, meget træt", but in some situations people say "Hold kæft hvor er jeg træt", which means the same (it is slang though and you will probably not see it in the written language very often).
Or "wow, she is beautiful" --> "hold kæft hvor er hun smuk!"
It is also considered to be swearing by some, which is why it is not often used in writing.