1. Informal a. A narcotic, especially an addictive narcotic. b. Narcotics considered as a group. c. An illicit drug, especially marijuana. 2. A narcotic preparation used to stimulate a racehorse. 3. Informal A stupid person; a dolt. 4. Informal Factual information, especially of a private nature. 5. Chemistry An absorbent or adsorbent material used in certain manufacturing processes, such as the nitroglycerin used in making dynamite. 6. A type of lacquer formerly used to protect, waterproof, and tauten the cloth surfaces of airplane wings. 7. Chiefly Southern U.S. See cola1. 8. Lower Northern U.S. Syrup or sweet sauce poured on ice cream.
The etymology of 'dope' quite surprised me:
Before it came to mean “a narcotic or narcotics considered as a group,” dope was borrowed into English from the Dutch word doop, “sauce.” Throughout the 19th century it meant “gravy.” In the North Midland United States, particularly Ohio, dope is still heard as the term for a topping for ice cream, such as syrup or a chocolate or fruit sauce. In the South, particularly in South Carolina, dope means “a cola-flavored soft drink.” The term might be related to the Northern usage as a reference to the sweet syrup base of a cola drink. However, folk wisdom has it that dope recalls the inclusion of minute amounts of cocaine in the original Atlanta recipe for Coca-Cola, which was named after this exotic ingredient.
The noun 'doop', meaning 'a thick sauce, gravy' doesn't exist in Modern Standard Dutch anymore. But 'dope' in the sense of stimulating narcotic (*) seems to go back to a secundary, technical meaning of that Dutch word, viz. a thick, sticky liquid that got used to absorb other liquids, and more specifically, at least according to one of the sources, a mixture of tar, oil, punpowder [edit: Gunpowder] and other stuff used in fireworks.
Modern Dutch reloaned English 'doping' as a noun, with the meaning of an illegal stimulating product. French 'doper' comes from AE 'to dope', while French 'dopage' also made it into Dutch, but it is now considered to be slightly dialectical.
Dutch 'doop' is related to English 'dip', and the normal word for 'to baptise' in Dutch is 'dopen', which seems to be due to Gothic missionaries (a.o. in Bavaria) who used 'daupjan' (related to OE diepan) as a translation of Greek 'baptizein' (which lit. means 'to dip').
(*) narcotic: from Mod.L., from Gk. narkosis, from narkoun "to benumb."
Hence my surprise at "narcotic preparation to stimulate a racehorse" -- or a Floyd Landis, or an Eddie Merckx. One of the cyclists in which both the stimulating and narcotic effect got united is Tom Simpson. (May he be eternally numb in peace).