I know it has probably been asked befor but try serching the archives for ‘The Staight Dope’ and you get a bunch of hits.
So why does having your dope striaght mean the information is correct?
Anyone want to place bets on how long it will take till someone (DuckDuckGoose) posts a link to the thread that answeres this question?
I would just like to point out that you managed to spell “straight” in three different ways throughout your OP. 
This thread addresses it and includes links to other threads which address the question.
Thanks for pointing that out.
I started thinking about this last night so I havn’t slept well worring about this all important subject and not completly awakened.
I don’t really like the horse racing answer and I have wondered a bit about how a stupid person could be a dope and information itself is dope. I seem to remember a process in building modle planes where you have to apply ‘dope’ to the wings after you covered them so I thought the term might have come from that.