"holy fuckin shit" is the adult version of "OMG". When a person is in complete awe or disbelief of a sensory input, this term is commonly used. It is a modified version of the much used "holy shit", only intensified by the versatile word fuck. It is usually used in extreme cases of all kinds.
What one would say when surprised, angered, or amazed by something. This is not blasphemous because you are not speaking of the Lord, just some dude with the same name, with explitives added for emphasis.