'Shit-boy' is a term to acknowledge surprise or shock to any situation. It can be used in an extensive range of circumstances, from mocking a friend that has done something trivial, to a response upon hearing the gravest of news. The tone of delivery, facial expressions and relationship are essential to the perception of sincerity/ use of the term.
A group of friends arrive at the airport to go on holiday.
"I forgot my passport"
Inhale and sympathetic.."..shit boy".
An outstandingly attractive group of girls walk past in the street.
A glance to your friend and a cheeky smile.."Shit-boy".