Drugs make the medical world go round; so, too, does dope.
Looking for the inside dope on dope? It's trucked a long and funny road. Beginning as doop around 1805, a Dutch sauce or gravy, from the verb dopen, "to dip," its consistency helped to name "a thick headed person" (c.1851). Its meaning expanded during the late 19th century when it identified a mixture of unknown ingredients; but by 1889 its definition had narrowed down primarily to "narcotics," mostly opium.
When trainers used narcotics on horses to fix a race, those with advanced knowledge of which horses had been drugged were said to have the inside dope. Together with the straight dope or "privileged information," which appeared twelve years later, it allowed us in 1906 to finally dope it all out.
How it got to be a drug on the market, however, has nothing to do with dope. It derives instead from the French drogue, "rubbish or poor stuff," describing a coarse material, half wool, half silk. Passed off as one hundred percent silk by unscrupulous dealers, its lower price initially drove down the sales price for the real stuff. Over time, however, it also diminished the demand for the cheap copy as well, the sheer quantity of the drogue on the market contributing to making both worthless — just so much junk for so many dopes.
Feeling duped by it all? No reason to. Dupe derives from the Middle French duppe (early 15thC) possibly from a reputedly stupid bird called a huppe... Huppe-de-do!