dope

From Dutch doop (“thick dipping sauce”), from Dutch dopen (“to dip”), from Middle Dutch dopen, from Old Dutch *dōpen, from Frankish *daupijan, from Proto-Germanic *daupijaną.

“Doop” in the sense “narcotic drug” ultimately refers to viscous opium juice, the drug of choice of the ancient Greeks; “insider information” perhaps from knowing which horse had been doped in a race. Sense of "stupid person" perhaps following from the drug sense (i.e. relating to those intoxicated on opium), compare dope up. Related to English dip and German taufen. Unrelated to dopamine.