Where does the term come from?
Did a Google search and found this http://home.t-online.de/home/toni.goeller/idiom_wm/idioms551.html#the straight dope BTW this looks like a great site.
Basically an extension of dope, which means to think about something (“doping something out”) which I recall reading in older books but not as much now.
It’s a stupid person who sets up punch lines …