in reference to being told something unbelievable or shocking, means to hold on and go back to what was just said; request to repeat and readdress a comment.
"Whoa. Back up the bus - what did you just say?"
"Hang on - back up the bus. You were doing WHAT last night?!"
"Hang on - back up the bus. You were doing WHAT last night?!"
by mypseudonym2012 September 26, 2011